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Biology 30
Module 3
Reproduction and Genetics
Lesson 12
Heredity and Genetics
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
Biology 30
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Lesson 12
Biology 30
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Lesson 12
Lesson 12 Heredity and Genetics
Directions for completing the lesson:
Text References for suggested reading:

Read BSCS: An Ecological Approach
Pages 163-167, 174-top 177, 184-187
OR
Nelson Biology
Pages 570-594

Study the instructional portion of the lesson.



Review the vocabulary list.
Do the practice Genetic problems.
Do Assignment 12.
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Lesson 12
Vocabulary
alleles
artifical insemination
cloning
condominance
dominant
embryo transplants
genetics
genotype
heredity
heterozygous
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homozygous
hybrid
inbreeding
incomplete dominance
phenotype
polyploidy
punnet square
recessive
recombinant DNA
110
Lesson 12
Lesson 12 – Heredity and Genetics
Introduction
The production of individual organisms from previously existing ones is discussed in
various areas of biology courses. Some forms of reproduction have new individuals
developing from some part of the body or cell of one parent. In these asexual forms
of reproduction, there are high degrees of similarities between parents and offspring.
Sexual reproduction takes place when there is a union of two cells, usually from two
different parts or parents, which then leads to the development of a new individual.
This reproductive technique also favors some similarities between parents and
offspring; however, variation is far more common in this method.
Up to now, descriptions of the different forms of reproduction has centered mainly on
the actions leading to the successful development of new individuals. Only general
and brief comments were made about the transmission or inheritance of
characteristics between parents and their offspring. Questions relating to this have
mainly been left unanswered. Some of these questions could be:
Why do offspring look like their parents?
Why can one offspring look different (hair colour, height, etc.) than all the others?
Why are there greater similarities between some parent-offspring sets than others?
Why do some offspring seem to suddenly appear with characteristics quite different
from those of their parents?
These next lessons will attempt to present understandings of how and why both
similarities and differences occur from generation to generation. In addition, current
understandings will also be applied on a wider scale to such areas as plant and
animal breeding, human genetics and changes over time.
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Lesson 12
After completing this lesson you should be able to:
•
distinguish between heredity and genetics.
•
state some of the early ideas about the transmission of traits
from parents to offspring.
•
state and explain the Mendelian Laws of Heredity, which are so
important as a starting point for genetic research.
•
use such terms as phenotype, genotype, dominant,
recessive, homozygous, heterozygous and alleles.
•
explain some of the laws of chance or probability.
•
use Punnett Squares in determining various genetic
probabilities.
•
explain the general purpose of test crosses.
•
determine probable results of monohybrid and dihybrid crosses.
•
describe reasons for, and situations involving, incomplete
dominance and codominance.
•
describe some techniques and developments in plant and
animal breeding including
·
·
·
·
·
·
·
·
·
•
•
•
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artificial and mass selction
inbreeding
outbreeding
hybrid crosses
polyploidy
artificial insemination
embryo transplants
cloning
recombinant DNA
explain the relationship between heredity and environment.
recall some human traits which follow the Mendelian laws or
principles of genetics.
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Lesson 12
Heredity and Genetics – Their Meanings
The two terms heredity and genetics are sometimes used interchangeably during
parent-offspring studies. However, for a person spending more time on such studies
or research, there is enough difference that the distinction between the two should be
noted.
Heredity is the passing of traits from parents to offspring. All dogs having certain
dog-like characteristics is an example of heredity.
One daughter strongly resembling her mother while another bears no resemblance
either to the mother or to the father are also examples of heredity.
Genetics is the study of heredity. A person studying the ways in which traits are
transmitted or trying to analyze the reasons for and the results of particular crosses,
is a geneticist.
Early Ideas of Inheritance
No doubt the questions of why members of a species resemble each other or why
family members may look similar or different have always interested people. No less
intriguing are questions related to the mechanisms by which such similarities or
differences are passed along or developed.
An early idea put forward by Aristotle about inheritance or heredity was based on
blood. This idea supposed that the blood of parents mixed and blended to result in
their offsprings' characteristics. Some of the terms still used today have their origins
in this idea: such terms are blood relatives, pureblood, blood lines.
The development of microscopes and discovery of eggs and sperm also started new
speculations.



One idea stated the existence of a completely formed individual, in very small size,
inside a sperm. Once implanted into a female body, this new individual just grew
in size.
Others felt that this complete individual was in the egg, rather than the sperm.
Another theory was that sperm and eggs contained sample cells from all body
areas and that, combined in embryos, they just reproduced those cells into
complete individuals.
The development and refinement of microscopes and the emergence of the Cell
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Lesson 12
Theory prepared the field of genetics for great advancements beginning in the early
1900's. The idea of cells coming from existing cells brought forward the idea of life
forms based on a continuation. That is, your cells originated from your parents,
theirs from their parents and, in this fashion back into time. The questions of how
far back and to what kinds of early life forms, often raise interesting speculations.
Surprisingly, the most important beginnings in genetics did not originate with
microscopic work or cellular studies. Instead, they began with simple breeding
experiments in which the experimenter had no previous knowledge of genes,
chromosomes or the processes of mitosis and meiosis and their roles in reproduction.
Mendel's Laws of Heredity
The works of Gregor Mendel (1822-1884), an Austrian teacher-monk, were
responsible for the beginnings of modern genetics. His educational background of
science and mathematical training enabled him to set up and carry out experiments,
to analyze and interpret the results and to use statistical information in meaningful
ways to draw important conclusions. The results of Mendel's experiments and
conclusions were published in 1866. As with some other great scientists, it was not
until some time after Mendel's death that his findings and conclusions led to
significant new expansions in the area of genetics. These began in the early 1900's,
approximately twenty years after Mendel's death.
Mendel's interest in genetics was centered primarily on finding out how
characteristics or traits were passed on from generation to generation. At this time,
probably the most common idea was based on the idea of blending of blood.
However, Mendel felt that reproduction and the passing on of characteristics was
carried out by special cells or gametes in all sexually reproducing organisms.
Accordingly, he felt that either animals or plants could be used in investigating
different ideas.
Mendel's breeding experiments were carried out mainly with pea plants. Likely, his
choice of this garden plant was decided by a number of favorable characteristics that
it possessed.
First of all, the pea plants were fairly easy to grow and had reasonably short life
cycles. This meant that results of any breeding experiments were determined
relatively quickly.
A second characteristic was the flower structure and shape of a pea is such that it
normally reproduces by self-pollination rather than cross-pollination. Mendel could
therefore let plants self-pollinate or, by removing stamens before maturity, he could
control reproduction by artificially pollinating them to carry out desired crosses.
Mendel focused on one trait at a time.
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Lesson 12
Another aspect of peas that made them suitable for experimentation was the
possession of some very distinctive characteristics or traits that were capable of
expressing themselves in opposing ways in different plants. For instance, one
characteristic or trait is that of stem length, where one plant could show either a long
stem or a short stem. Unlike other plant and animal breeding experimenters of the
time, Mendel selected and used only a few traits for his crossing trials. The seven
traits he focused on are shown in the illustration below.
Mendel initially began his trials with pure breeding parents
for the characteristics or traits described. He took pure
breeding plants for one particular trait and crossed them
with pure breeding plants having the opposite trait. The type
of experiment Mendel carried out with one specific trait is
described next.
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Pure breeding plants
produce offspring that are
identical to themselves
generation after
generation.
Lesson 12
Plants with tall stems were crossed with short stemmed
plants. When the seeds from this cross were planted Mendel
found that all of the offspring grew as tall as the tall parent
plant. These offspring are referred to as the first filial, filial
one or F1, generation of plants.
The term filial is derived
from a word that refers
to offspring, or
sons/daughters.
Next Mendel allowed individuals of the F1 generation to pollinate themselves, He
found a reappearance of some short-stemmed individuals, along with individuals
having tall stems. These offspring were referred to as the second filial or F2
generation.
With repeated testing of a large number of plants and careful counting, an
approximate 3 to 1 ratio of tall stems to short stems was noticed in all F2 generations
from originally "pure" parental types (generally designated as P1).
INSERT IMAGE HERE
Mendel did monhybrid crosses (one trait crossed at a time) with seven traits of the
pea plants. For example - Plants having yellow seeds were crossed with those having
green seeds. Continued experimentation showed similar F1 and F2 results.
INSERT RESULTS HERE
Applying his knowledge of mathematics to the results of the crosses Mendel was able
to put forward some very important conclusions.
1. His first important hypothesis was the idea of Unit Characters.
The idea of unit characters stated that traits were controlled by
pairs of "factors" (or genes, as we know them now), with one
factor coming from each parent. The alternative forms of the
gene are called alleles eg For height there was tall and short, for
seed colour there was green and yellow.
It should be kept in mind that Mendel knew nothing about chromosomes or the
process of meiosis that separates similar pairs when gametes (eggs and sperm)
are produced.
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Lesson 12
2. The results of crossing two pure breeding parents for contrasting traits led to
another important conclusion. Many geneticists of the time believed in the
blending of parents' traits. However, Mendel's F1 generations consistently had
individuals showing only one of the two contrasting traits possessed by the
parents. F1 individuals resulting from tall stem and short stem crosses were all
tall, rather than intermediate in size.
This led to the principle of dominance and
recessiveness that stated that one factor (gene) in a
pair may mask the other, or prevent it from having an
effect.
Mendel called genes that were “stronger” or which covered the effects of others,
dominant genes. The genes that were masked were called recessive.
An individual resulting from a cross between pure parents for contrasting traits
and therefore having dissimilar pairs of genes is said to be a hybrid.
3. The reappearance of some short stemmed individuals in the F2 generation led to
the Law of Segregation: members of a pair of factors
(genes) are separated, or segregated, during the
formation of gametes.
One gamete receives one factor or gene, while another gamete receives the other.
4. Sometimes combined with the Law of Segregation, but usually standing
separately, is another hypothesis by Mendel called the Law of Independent
Assortment.
Mendel felt that different gene pairs separated and
were distributed to gametes independently of each
other.
For example, a pair of genes for stem length separate independently of another
pair affecting flower color.
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Lesson 12
How Mendel’s Laws Apply
To have a better understanding of the Mendelian laws and how they apply, one of
Mendel's trial crosses could be followed through more closely.
Common genetic terminology will be used and followed in this examination of a cross
between a pure breeding tall plant and a pure breeding short stemmed plant. The
original parents used in any particular cross are designated as P1. If a plant is pure
breeding, the genes of a gene pair governing that trait are the same. Mendel and the
geneticists who followed him identified the genes in short form by using just their
first letters.


The first letter of a dominant gene was usually chosen and it was expressed in
capitalized form.
The recessive gene was expressed by the lower case letter of the dominant gene.
So, the pure breeding tall stemmed individuals were identified as TT.
The pure breeding short stemmed, were identified as tt.
This first parent cross is shown below.
P1
TT
(pure tall)

tt
(pure short)
Production of gametes (other names for gametes are sex cells, or eggs and sperm) by
each of the preceding parent types sees a splitting of the gene pairs. Since the genes
of each parent's pair are the same, each individual can only produce gametes
carrying the gene for one trait.
With each parent producing only one type of gamete, the cross could be simplified
and shown as:
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Lesson 12
Geneticists commonly use another set of terms to describe gene pairings.
Homozygous describes a pairing where the
genes are the same.
Heterozygous refers to a pairing of contrasting
or different genes.
In the example so far, the original parents are homozygous tall (TT) and homozygous
short (tt). The F1 generation is all heterozygous tall (Tt). The results of this cross
illustrate the Mendelian principle of dominance. Allowing the F1 generation to
self-pollinate produces the following type of cross: (This is called an F1 cross)
The actions leading up to and producing the second filial or F2 generation gave rise to
Mendel's Law of Segregation.
The external appearance or the outward effect that a dominant gene
produces gives an organism its phenotype. The phenotype of all the
F1 generation is tall. The combinations of TT and Tt in the F2 again
result in the tall phenotype. The tt pairing produces a short-stemmed
phenotype. In terms of total numbers, there would be an approximate
3:1 ratio of tall to short plants.
The term genotype is used in describing the actual genetic
composition of a pairing. The F2 generation shows three possible
genotypes: TT (homozygous tall), Tt (heterozygous tall) and tt
(homozygous short). In terms of probabilities or numbers, this would
work out to a
l:2:1 ratio.
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Lesson 12
The Role of Meiosis in Reproduction
Although Mendel had no knowledge of chromosomes and meiosis, the
reduction-division process is an important part in sexual reproduction. In a way,
Mendel's Law of Segregation really describes meiosis. The members of pairs of
chromosomes in parent body cells are split up. Chromosome numbers are reduced
by one-half, or to the haploid numbers, in gametes (eggs and sperm). This prevents a
doubling of chromosomes in generation after generation of offspring.
As genes are located on the chromosomes, splitting of homologous chromosome pairs
also separates gene pairs. As a result, a gamete normally ends up with one gene
from each gene pair.
Laws of Chance or Probability
In the earlier description of a cross between two heterozygous long-stemmed plants
(Tt), one may have received the impression that four offspring were produced. Actual
offspring numbers vary. The results were used to show probability of the kinds of
results that may occur.
If four offspring were indeed produced, they could
all have had just one of the particular genotypes
mentioned or combinations of other genotypes.
However, whatever the number of offspring, it is more likely that genotypes would
follow the 1:2:1 ratio.
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Lesson 12
The laws of chance are based on mathematical formulas used to predict the chances
or probabilities of events happening. Tossing a coin in the air has an equal
probability, or a 50:50 chance, of resulting in a head or a tail.
In mathematics there is a multiplication principle for calculating the chances of two
separate events occurring together. For example, in tossing two coins together, one
can calculate the chances for various combinations such as two heads appearing
together.
The multiplication principle states that the probability of two events occurring
together is equal to the probability of one event occurring alone multiplied by the
probability of the other event occurring alone.
In numbers:
1 1
1
 or
2 2
4
There is one chance in four of two heads
occurring simultaneously.
The same reasoning and calculation applies in using just one coin and trying to
predict ahead of time the probabilities for certain events for a number of trials.
The calculated chances (ahead of time) of tossing one coin three times and having
heads on all occasions would be
1 1 1
1
  or
2 2 2
8
There would be one chance in eight attempts for arriving at this result .
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Lesson 12
The concept of Independent Events states that the outcome of a previous event has
no effect on the next one(s). Before tossing a coin three times, a quick calculation
will indicate that there is a one out of eight (1/8) chance of getting all heads. (As
calculated previously)
Question: Toss the coin a fourth time. What are the chances of
getting heads?
Answer:
The concept of Independent Events would treat the next toss
as a single event. The chances of getting heads would be
50:50, or 1/2.
Applying Probability to Genetic Crosses
In using the laws of probability, one can predict the possible chances of certain
genetic crosses taking place. Applying this to a Tt × Tt cross, one can come up with
the following types of calculations for various combinations:
a.
Chances of having a TT offspring from the following cross:
Tt (female)
×
There is a ½ or 50:50 chance
that an egg will carry the T
gene.
Tt (male)
There is a ½ or 50:50 chance
that a sperm will contain the
T gene.
There would be a ½ × ½ or ¼, or a one in four chance of a TT
individual resulting.
One can use similar reasoning and calculation to arrive at the one in four chance for tt.
b.
Chances of having a Tt in the offspring from the same cross:
The combination of Tt in the offspring can be looked at a little differently. A T
from the female combining with a t from the male has a ¼ probability. Also, a
T from the male and a t from the female has the same ¼ probability. The
genotypes and phenotypes are the same, however, in being Tt and tall, so that
the two could be combined or added: 14  14  2 4 or 12 . There would be a 50:50
chance or ½ probability of having a Tt offspring in such a cross.
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Lesson 12
Punnett Squares
To work out possible combinations and ratios of different crosses, a British
mathematician and biologist devised the use of tables (named after him) for
calculations. The system consists of drawing up squares with two of the sides
representing all the possible gamete-gene combinations of the parents.
Example 1
A Tt x Tt cross can be worked out in the following manner:
Each parent can produce two possible types of gametes, either T or t. The Punnett
Square is drawn up with two sides showing the possible gametes of the two parents.
Determining the possible genotypes is then accomplished by drawing into each
square the gamete of each parent.
Each square represents a probability of a certain combination happening. Similar
combinations could be united.
A Punnett Square can then be used to summarize:
1. The genotypic ratio. This type of ratio represents all the possible different
genotypes of a cross.
The genotypic ratio of the preceding cross would be: 1 TT: 2 Tt: 1 tt.
In other words, this could be expressed as one homozygous tall to two
heterozygous tall to one homozygous short. (Note that the words heterozygous and
homozygous indicate genotype.) In numbers, this ratio could be expressed as: 25% will be
TT: 50% will be Tt: 25% will be tt.
2. The phenotypic ratio represents what all the possible offspring will look like.
In the example above, the phenotypic ratio is 3 tall to 1 short.
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Lesson 12
Example 2
Another trait Mendel identified was the pod color. Green pods (G) were dominant to
yellow pods. Determine the genotypic and phenotypic ratios if the parent cross P1 
is Gg  Gg .
Solution:
P1
Gg × Gg
Each parent can produce two possible types of gametes, a big G or a little g. Draw a
Punnett Square to determine all possible genotypes of the offspring.
Summarize the results:
The genotypic ratio is:
The phenotypic ratio is:
1GG : 2Gg : 1gg
3 green pods : 1 yellow
Test Crosses
In plant and animal breeding experiments or in actual breeding operations, some
uncertainties could exist about the genotypes of particular individuals. The actual
genetic makeups or genotypes of individuals displaying recessive traits are easy to
identify.
For a recessive trait or characteristic to appear, the genes
must be in a homozygous condition (gg or tt).
The genotype of a short-stemmed pea plant has to be tt.
If you have a pea plant that has a phenotype of tall, you can not be certain of the
genotype. The genotype of a tall stemmed pea plant can be either homozygous (TT)
tall or heterozygous (Tt) tall.
To identify the "pureness" or actual genotypes of some individuals showing dominant
traits, breeders can carry out identifying test crosses. A test cross is best used for
individuals having multiple offspring. Test crosses are sometimes preformed to
determine the genotype of a dominant phenotype. The cross involves the use of one
parent that is a homozygous recessive with the parent of the questionable genotype.
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Lesson 12
Example:
A biology student was given a tall stemmed pea plant. The student was
asked to identify the genotype of that particular pea plant. How would
the student proceed?
Step 1 – The student writes down the possible genotypes for the tall
stemmed pea plant.
The possibilities are: TT or Tt
Step 2 – The student chooses to do a test cross with a pea plant of which
he/she knows the genotype. The cross that is chosen is:
The short stemmed pea plant
tt
(Known genotype)
×
×
the tall pea plant
TT or Tt ?
(Unknown genotype)
Step 3 – The student can attempt to predict the possible outcomes using
a punnett square. The Punnett Square of the first cross would be:
tt × TT
Possible outcomes: All offspring would have a genotype of Tt. The phenotype would be
all tall.
The Punnett Square of the second cross would be:
tt × Tt
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Lesson 12
Possible outcomes:
50%
50%
50%
50%
of
of
of
of
the
the
the
the
offspring
offspring
offspring
offspring
would
would
would
would
have
have
have
have
a
a
a
a
genotype of Tt
genotype of tt.
phenotype of tall.
phenotype of short.
Conclusion: After both crosses are done, if any of the offspring are short (tt) the
student will know that the genotype of the tall pea plant is Tt and will no longer cross
those particular seeds if they don’t want short plants.
A test cross is more practical to carry out where there are a lot of offspring or there
are multiple births. The results are likely to be more reliable and conclusive as
compared to individuals having few or only one, offspring at a time.
Monohybrid and Dihybrid Crosses
Up to this point, the examples of some of the different crosses have involved the
examination of only one particular trait at a time, or only one pair of genes.
Monohybrid crosses include those where different genes for one (mono) trait come
together to produce a hybrid or heterozygote, as in TT × tt to produce Tt. A
monohybrid cross can also refer to the crossing of two heterozygous (for one trait)
individuals, as with Tt × Tt.
Initially concentrating on only one particular characteristic enabled Mendel and other
geneticists, then and now, to interpret results more readily. However, the numbers of
chromosomes and the numbers of genes in individual species are seldom limited to
one pair.
Offspring that result from a dihybrid cross are heterozygous for two (di) different
traits (two pairs of genes), as in TTRR × ttrr to produce TtRr. A dihybrid cross can
also include crosses between TtRr individuals. Such a situation could be that of two
individual pea plants heterozygous for length of stem and also type of seed as in
TtRr × TtRr. R represents a (dominant) phenotype for round seed while r stands for
the recessive gene that produces a wrinkled seed. (T represents tall (dominant) while
t stands for short, the recessive gene.)
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Lesson 12
An example of the first type of dihybrid cross involving homozygous parents. In
crossing a homozygous tall stemmed, round seed plant with a homozygous short
stemmed, wrinkled seed plant, the initial cross should not be difficult to follow. Each
parent produces gametes (egg and sperm) which show only one possible combination.
The TTRR parent's gametes will all be TR while those of ttrr are tr.
T = dominant tall
t = recesive short
R = (dominant) round seed
r = recessive a wrinkled seed.
Genotypes of parents:
homozygous tall stemmed round seed plant : TTRR
homozygous short stemmed, wrinkled seed plant : ttrr
The gamete has to have one
piece of information from
each trait. In this case, the 1st
parent has T on both genes so
all gametes will have a T. The same with
R. All gametes from the 1st parent are TR
The gamete has to have
one piece of information
from each trait. In this
case, the 2nd parent has
t on both genes so all
gametes will have a t.
The same with r. All
gametes from the 2nd
parent are tr.
The resulting offspring in the F1 (filial one) generation will all show the genotype
TtRr. The phenotypes of all F1 individuals are tall stemmed with round seeds.
An example of the second type of dihybrid cross involves heterozygous parents.
Continuing on from the previous example, if we do an F1 cross we have an example
of a dihybrid cross involving heterozygous parents.
The F1 cross is:
TtRr
×
TtRr
Step #1- You need to determine all possible gametes (or sex cells or eggs and/or
sperm) that each Parent will have. An easy way to determine this is as
follows:
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Lesson 12
Parent #1
Possible gametes
To determine the gametes of Parent 1, place
a dot above the 1st big T, draw
an arrow to the first gene(R) of the
2nd trait. Number it 1 and record this gamete TR.
Now, go back to the dot above the T and draw an
arrow over the 2nd gene(r) of the 2nd trait. Number it 2
and record this gamete as Tr (see to the right)
1. TR
2. Tr
Possible gametes
Next put a dot under the little t and draw an arrow
to the first gene (R)of the 2nd trait. Number it 3.
Record this gamete as tR. Go back to the dot under
the t and draw an arrow from the dot over to the 2nd
gene (r) of the 2nd trait. Number it 4 and record the
gamete as tr.
3. tR
4. tr
These are all possible gametes for the first parent.
Parent # 2
In this cross, the genotype of the second parent (TtRr) is the same as the first parent
so the possible gametes would be the same as the first parent: TR, Tr, tR, tr.
If the genotype of the second parent is different than the first parent then you must go
through the same process as for parent #1 to find all the possible gametes. (Remember
in place of the word gamete you can also use the words sex cells or eggs and/or sperm)
Step # 2 - Determine the genotypes and phenotypes of the offspring from this
dihybrid cross. To do this, use a Punnett Square. The number of
squares on each side would match the number of possible gamete
combinations. In a dihybrid cross, that makes this a four by four
Punnett Square.
1.
Place the gametes in the Punnett Square
TR
Tr
tR
tr
TR
Tr
tR
tr
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Lesson 12
2.
Fill in the squares. (As the two gametes meet it is like fertilization occurring
and a zygote forming – the chromosome number is restored to 2n, whether it is
in plants or animals).
Always keep the first trait (T or t) on the left throughout the whole Square.
Why? Because it provides order and makes it easier to identify the genotypes
and phenotypes.
TR
Tr
tR
tr
TR
TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
Step # 3 - Identify the phenotypes by going through each square.
Phenotypes
a.
The genotype in the first square is TTRR, the second square is TTRr etc. The
phenotype is – tall plant and round seeds. Count all the phenotypes that are
tall and round. (See shading)
IIII IIII = 9
Biology 30
TR
Tr
tR
tr
TR
TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
129
Lesson 12
b.
The second phenotype is tall and wrinkled. It is represented by the genotypes
TTrr and Ttrr. Count the number. (See shading)
III = 3
c.
TR
Tr
tR
tr
TR
TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
The third phenotype is short and round. It is represented by the genotypes
ttRR and ttRr. Count the number. (See shading)
III = 3
Biology 30
TR
Tr
tR
tr
TR
TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
130
Lesson 12
d.
The fourth phenotype is short and wrinkled. It is represented by the genotype
ttrr. Count the number. (See shading)
I = 1
TR
Tr
tR
tr
TR
TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
Step # 4 - Write the ratio of the phenotypes called the phenotypic ratio.
9 tall, round: 3 tall, wrinkled: 3 short, round: 1 short, wrinkled
These numbers represent probabilities or possibilities. To get this type of ratio you
need to have large numbers of offspring. A one time mating may not yield a ratio that
fits this.
The results of dihybrid crosses led Mendel to his principle of Independent
Assortment. This concluded that factors (genes) for one trait have no effect on how
factors (genes) for another trait separate and assort themselves into gametes. This
was true for all of the traits of the pea plant that Mendel studied. The principle also
holds true for many other traits of plants as well as other organisms. However, as
shall be seen later, this is not exclusively true for all traits.
Using the Principles of Segregation and Independent Assortment, a person can go on
to trihybrid or other polyhybrid crosses. Crossing two individuals of RrYyCc
genotypes would require a larger Punnett square. The number of gamete
combinations possible is eight. Therefore, the total number of squares would be 64.
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How can we be sure that you’ve gotten all the possible gametes?
You can do a calculation to determine how many different gametes there will be when
doing a dihybrid, trihybrid, polyhybrid or even a monhybrid cross. The calculation is
2n, where the n refers to the number of heterozygous gene pairs (simply put the
number of traits involved).
For example:
For a monohybrid cross - the number of traits involved is 1. So 2n is 21 = 2 possible gametes.
For a dihybrid cross – the number of traits involved is 2 (di). So 2n is 22 = 4 possible gametes.
For a trihybrid cross – the number of traits involved is 3 (tri). So 2n is 23 = 8 possible gametes.
This mathematical relationship is fairly accurate if different gene pairs are on
different chromosomes; however, many different gene pairs are on the same
chromosomes and the results of some crosses are not as easy to summarize or to
predict. Although the Principle of Independent Assortment still applies to
chromosomes themselves, it can no longer include all genes. This will be examined
more closely when gene linkage is considered.
Codominance and Incomplete Dominance
Some of Mendel's results with crossing peas led to his Law of Dominance. This
stated that when a pair of genes for the same trait has different alleles, one of the
genes expresses itself over the other, for example Tt where T (tall) is dominant over t
(short). The dominant gene is expressed, while the recessive gene is covered up.
Thus, the phenotype (appearance) of a Tt pea plant is tall.
Incomplete Dominance
Later works by other scientists found exceptions to the dominant - recessive
relationship normally found between most gene pairs. Crosses between red and
white-flowered snapdragons produced plants having pink flowers. Crosses of early
flowering plant varieties with late flowering ones produced intermediate flowering
offspring. In a variety of chickens, the Andalusian fowl, black males mated with
white females produced offspring having a gray-like color, which appears blue from a
distance. These exceptions to the normal dominant-recessive nature of gene pairs
are examples of incomplete dominance. Effects of some heterozygous gene pairs
produce blending effects in the phenotypes of offspring. Such blending results could
have inspired some of the earlier scientists of Mendel's time and before, who believed
that offspring characteristics were inherited through a blending of their parents'
blood or traits. However, they would have found it difficult to explain the
reappearance of the original parent traits in some of the offspring of the second
generations (from "blended" parent crosses).
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There is some lack of uniformity among reference sources in the manner of
identifying and working with incomplete dominance. Some use the capital letter of
one of the parent phenotypes and then identify the contrasting phenotype with the
same letter but with a dash or a one beside it. For example, a red flower could be
identified by R and a white flower by R1. Other references use lowercase letters of
both phenotypes: r for red and w for white. The use of different letters may be better,
as it gives the idea of the equal strength of the two genes and the blending effect they
produce. This course adopts the use of different letters for codominant genes, but
they will be used in capital form. Crosses between red and white snapdragons can be
shown in the following manner:
Mendel's Law of Segregation becomes apparent when crossing two heterozygous
individuals. The separation of genes and then their recombinations in various ways
during reproduction results in a 1 : 2 : 1 ratio of red to pink to white. Results of
crossing two pinks can perhaps be seen better with the use of a Punnett Square.
RW
X
RW
R
W
R
RR
RW
W
RW
WW
Genotype ratio:
1RR : 2RW : 1WW
Phenotype ratio:
1 red : 2 roan : 1 white
Codominance
Incomplete dominance occurs when contrasting
genes of a pair produc e a blending effect in the
offspring. A somewhat similar situation exists with
codominance. There is a slight difference in that,
rather than having the original characteristics give
way to an intermediate effect or a blending effect,
those original characteristics remain and mix
together. Codominance is probably best seen in the
roan colors of some animal hairs or coats.
Shorthorn cattle commonly show this feature.
Crossing a "red" and a white animal produces a
roan offspring, where red and white hairs are mixed
together. Blood type AB is another example.
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Lesson 12
Human Traits Following Mendelian Principles
It is very interesting to look at and compare physical traits that are present in
humans. Below are several traits that are common. Have some fun. Check out which
traits, and see which you possess and determine whether they are dominant or
recessive. You may want to check out other family members. Record your “results” in
the chart at the end of the physical traits.
Widow’s Peak
Pull the hair back on your forehead. A distinct downturn or V-shaped point of hair is
dominant to a straight hairline which is homozygous recessive.
INSERT IMAGE HERE
Tongue roller (Dominant)
Try rolling your tongue. If you can roll your tongue you possess the dominant trait.
If you cannot roll your tongue you have the two recessive genes.
Image by Gideon Tsang
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Cleft chin (Dominant)
If you have a dimple in the midline of your chin, you possess the dominant trait for
that trait. The depth of the "dimple" if it is present, varies from person to person.
Unattached earlobe (Dominant)
A free or unattached earlobe is dominant to the attached condition.
Mid-digital hair (Dominant)
The complete absence of hair in the mid areas of fingers are recessive conditions.
The presence of even one hair on one of the digits indicates a dominant condition.
The number of hairs and the number of digits affected are determined by the number
of dominant genes.
Bent Vs straight little finger (Dominant)
With the palms of your hands facing you, put your two little fingers together. If the
tips of your little fingers point away from each other, then your little fingers are bent.
See diagram above. This the dominant trait. If your little fingers are straight you
possess the recessive trait. ).
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Lesson 12
Hand clasp
When clasping both hands together, with no thought of finger arrangement, most
people will fold their fingers in a consistent manner that feels natural to them.
Trying the other way causes an unnatural feeling. Left thumb and fingers over right
thumb and fingers is dominant.
Hitchhiker's thumb
The ability to bend the tip of the thumb so that it forms about a 45 angle, or greater,
with the rest of the thumb is a (homozygous) recessive trait. A straight thumb is the
dominant trait.
Image by Manicrage
Big Toe Length (Hallux Length)
image by Vaikunda Raja
Check your big toe. Compare its length to your second toe. A big toe that is shorter in
length compared to the second toe is a dominant trait. The homozygous recessive
trait shows the big toe being longer or equal in length to the second toe.
image by Michiel1972
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Lesson 12
Red hair (Recessive)
Red hair is typically recessive to browns and other colors.
Freckles
Freckles are dominant to not having freckles.
Long eyelashes
Long eyelashes are dominant to short eyelashes.
Hair whorl
Check the back of your head, If your hair whorl rotates clockwise you possess the
dominant trait. Counterclockwise is recessive.
PTC tasting
The ability to taste a particular chemical compound (phenylthiocarbamide), which
has a bitter taste, is a dominant trait.
Trait
Dominant Trait
Recessive Trait
Widow’s Peak
V-point
straight hairline
Tongue Roller
can roll
can’t roll
Cleft Chin
have dimple in midline
of chin
no dimple in chin
Ear Lobe
unattached
attached
Mid-digital hair
have hair
no hair
Bent little finger
have bent little finger
straight
Hand Clasp
left thumb & fingers
over right
no left thumb over
right
Hitchhiker’s thumb
straight thumb
bent thumb
Big toe Length
big toe shorter
big toe longer or of
equal length
Red hair
brown and other
colors
red hair
Freckles
freckles
no freckles
long eyelashes
short eyelashes
Hair whorl
clockwise whorl
counter clockwise
whorl
PTC tasting
can taste
can’t taste
Long eyelashes
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Your Data
Lesson 12
Plant and Animal Breeding Techniques
Heredity and the field of genetics are good illustrations of differences between pure
science and applied science. Mendel's findings and those of many others
contributed to an increasing body of pure knowledge or information about the
natures of chromosomes and genes and their actions. Applied science or
biotechnology, in the sense of using knowledge for practical, everyday situations, has
been following close behind in the footsteps of the pure scientists. People have been
using genetics almost continuously in attempting to develop better plants and
animals or to introduce new varieties. Some of the techniques in use for practical
purposes will be examined briefly here.
Selective Breeding
Through the years breeders have chosen plants and
animals that have ‘desired characteristics’ to cross
and produce the next generation. Some of the early
desired traits in plants were: rust resistant wheat,
more kernels in a head of wheat to give a higher
harvest, sweet corn, greater milk production, juicier
berries, etc.
Breeders want each plant or each animal to
consistently have the desired characteristics. To
achieve this consistency it takes many generations of
breeding. This has led to an increased frequency of
the desired alleles within a specific population.
Selective breeding is the essence of genetic
technology.
Image by Tarquin
Example
A Siamese Cat has
consistent desired
characteristics.
Image by Trinny True
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Lesson 12
Inbreeding (Line breeding)
A form of controlled breeding crosses closely related individuals over a number of
generations. With many plants and animals, these crosses are usually between
brothers and sisters. The intent of this type of breeding program is to try and
establish "pure lines", where individuals will eventually be homozygous for certain
desired traits. Individuals will "breed true", in that offspring will likely continue to
show particular distinctive traits possessed by their parents or breed. At the same
time, variations are minimized. As examples, breeders have developed pure breeds in
dogs and horses.
Disadvantages to Inbreeding
Image by LillyM
Inbreeding has some disadvantages, especially if carried on over a number of
generations. Trying to establish homozygous conditions for some desirable
characteristics is frequently accompanied by the same kind of result for some
harmful traits. These harmful recessive genes, which were originally masked by
dominants in earlier generations, could begin to appear in homozygous condition.
Certain structural weaknesses, health defects and losses in fertility could become
more and more apparent as inbreeding programs continue. Some breeders will try to
counter this by introducing an outcross into one of the generations. In an outcross,
an inbred organism is crossed with another organism of the same breed or variety,
but one that is unrelated. This has the effect of introducing "new" genes to possibly
counter effects of bad ones.
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Lesson 12
Crossbreeding or Hybridization
While inbreeding programs use individuals that are closely related, crossbreeding
takes a different approach. Individuals of different varieties or breeds, but of the
same species, are crossed. Crossbreeding is quite commonly seen on prairie farms or
ranches in cattle, hog or sheep operations. Producers in such operations will
introduce a male animal or sire of a breed that is entirely different from that of the
herd or flock of females. Charolais, Simmental, Limousin or other breed bulls are
common sights in established herds of breeds different from themselves.
The offspring of plants or animals in crossbreeding are often called hybrids, although
this term really applies to crosses between different species. Most individuals
resulting from crosses will show a general characteristic called hybrid vigor, in
which the offspring tend to be stronger or more "vigorous" than either of the parent
types in a number of ways. Hybrid vigor develops as a result of different dominant
genes coming from both parent types and combining in the offspring. The presence
of more of these dominant genes enables offspring to generally outperform their
parents in growth, development or other characteristics.
Hybridization could be carried out with two varieties or breeds only. However, many
crossbreeding programs today are based on more than two varieties. A certain corn
hybrid is developed using four different varieties. Using general letters, varieties A
and B are crossed to produce a hybrid and another hybrid is developed from C and
D. Then, the two resulting hybrids are crossed to produce still another hybrid. Corn
growers may then sell this particular hybrid commercially for use as seed.
In carrying out crosses similar to this, or in other ways, breeders attempt to combine
desirable features from all the varieties involved. The disadvantage of using some of
the commercially produced hybrids is that growers or producers are not advised to
use the next generation seeds or animals for further reproduction. To do so could
result in all sorts of variations between all the original parent types used. A producer
would not be sure of what to expect from future generations.
The term hybrid more correctly applies to another
type of situation. In the preceding descriptions, it
was applied to crosses between different varieties
or breeds within the same species. The term
applies more to crosses between entirely different
species. The familiar case of crossing a female
horse and a male donkey to produce the hybrid
mule is a good example.
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Lesson 12
Another example is that of the domestic cattle and North American bison cross to
produce the beefalo. The major shortcoming of these types of hybridization is that
hybrids are usually sterile. Different species have different chromosome numbers.
While these may be able to unite and result in successful fertilizations to form
hybrids, the unmatched sets cannot pair up during meiosis in the hybrids to produce
fertile eggs or sperm.
Polyploidy
In other sections of the course, the terms haploid and diploid are mentioned in
connection with chromosome numbers. The normal chromosome number in plant
and animal body cells is diploid. That is, chromosomes exist as similar or
homologous pairs, with one of each pair coming from both parents. Following
meiosis (the formation of eggs and sperm), the chromosome number is reduced to
1/2of the original number, the haploid number.
In various genetic experiments, scientists have discovered chemicals or techniques to
alter the normal process of meiosis. One such chemical, colchicine, prevents spindle
formation in the division process. Without this spindle, chromosome pairs will not be
separated. As a result, gametes can be produced having the diploid or 2n
chromosome number instead of the normal haploid or n condition. The union of
such gametes can produce some zygotes and individuals with a 4n or tetraploid
chromosome number. 3n or 5n individuals are also possible.
Polyploid conditions can occur naturally in some plants. There are species of the
coffee plant that has 22, 44, and higher chromosomes. It is thought that the original
chromosome number was 22 (2n). Another example is wheat, which is thought to be
hexaploid with a chromosome number of 42.
Whether they are natural or induced by humans, some polyploid plants exhibit more
vigorous growth patterns than normal diploids. Faster growth rates, a general
increase in hardiness, more flowers and larger seeds or fruits are some of the
possible outcomes of polyploidy.
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Lesson 12
Cloning
In Plants
Cloning is a form of vegetative or asexual reproduction. A clone or new organism is
derived from some part of the body of another individual. Genetically, clones are the
same in chromosome and gene composition as their parents. In general, clones can
result from such actions as cell fission (bacteria), budding (yeasts), rhizomes
(quackgrass), tubers (potatoes), bulbs (onions) or regeneration (earthworms).
Humans have encouraged or modified some of the natural cloning techniques. New
growths can be started by layering or covering certain plant parts with soil or water,
by fragmenting or cutting rhizomes or tubers into smaller pieces and by the use of
slips, where pieces of stems or leaves are started in good growing media. Cloning in
plants has been successful.
Scientists have been able to remove small numbers of cells from parts of some plants,
place them in artificial media with certain kinds of nutrients and hormones and have
them grow into new individuals. Sometimes, the term tissue culturing is applied to
this technique.
The difficulty in plant cloning has been to find the right kinds of artificial media and
hormones to start growths. Different plant tissues generally require different media
(nutrients) and hormones. There are differences in exact techniques as well. For
instance, some clones can be started from older plant tissues while other clones are
only possible from young, embryonic cells.
Plant cloning is most useful for reproducing large numbers
of desirable plants in much reduced periods of time. In
addition, by choosing healthy cells and reproducing them
under controlled conditions, virus free individuals can be
established.
In Amphibians
Cloning has been accomplished in amphibians. Frog clones have been produced by
taking nuclei from the body cells of the first frog and inserting them in the place of
the nuclei in egg cells of the second frog. These egg cells would then develop into
individuals having the same genetic characteristics as the first frog.
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Lesson 12
In Mammals
This method of nuclei transfer done in amphibians has also occurred in mice.
Cloning has successfully occurred in sheep. Read the following article.
July 5, 1996 made history, as cloning
was successful in a larger mammal.
Dolly, the Sheep, was successfully
cloned. Scientists took a nucleus from a
mammary gland cell of a Finn Dorsett
sheep and transplanted it into an egg
cell of a Scottish blackface ewe that had
the nucleus removed. The two cells were
fused and stimulated to cell division
with an electric current.
As time progressed and Dolly grew, a
fear that was felt amongst the
scientists was that Dolly might be
prematurely old. By 1999 Dolly’s
body cells were showing signs of being
more like an older animal. Dolly was
bred normally and did give birth to
four lambs between 1998 and 1999.
On Fri. Feb 14, 2003 Dolly was put to
death due to premature aging and
disease.
The new dividing cell (embryo) was
placed in the uterus of the blackface
ewe. Dolly was born months later. She
was genetically identical to the Finn
Dorsett mammary cells.
The following diagram shows the process used to clone “Dolly”.
Insert diagram here.
Link to creative commons image:
http://en.wikipedia.org/wiki/File:Dolly_clone.svg
This has raised ethical questions about the viability of cloning life. There are many
animal clones throughout the world. They are found in cows, pigs, mice and goats.
There has even been a claim that a human was cloned in January of 2003. This has
not been verified as of yet.
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Lesson 12
Artificial Insemination
The practise of collecting semen (containing sperm) from males having some superior
or desirable traits and then implanting many females with it has been going on for
some time now. Beef, dairy, swine and other livestock producers have been able to
upgrade or introduce desirable traits to their animals at reasonable costs. The ability
to successfully store vials of frozen sperm at low temperatures has enabled some
sires to keep fathering offspring long after their own deaths.
The use of sperm banks and artificial insemination is also used with people. In some
instances where human females have defects in their reproductive tracts, eggs are
removed, fertilized externally and then implanted back into the females to complete
development.
Embryo Transplants
In beef or dairy operations, most female cows average seven to eight offspring in their
lifetimes. Another reproductive technique has created the possibility of one female
producing 20 to 30 and perhaps more offspring annually. Using hormones, a female
can be made to superovulate or produce many eggs during each of her reproductive
cycles. After fertilization is carried out and some development has taken place, the
embryos can be surgically removed or "flushed out" of this donor animal. The eggs
can also be flushed and fertilized externally (in vitro) and allowed to begin dividing.
Each embryo could then be implanted into another female for development to be
completed.
Techniques have been developed so that each embryo's sex can be determined before
implanting into the carrier or surrogate mother. Removing several cells from an
embryo and developing karyotypes allows for microscopic examinations of the
chromosomes. These not only determine whether the embryo is female or male, but
may also reveal other things, including abnormalities.
Besides producing many more offspring from individual females, there are other
possible benefits from the use of this technique. They are:


Embryo transplants could be used to create disease free individuals.
Special nutrient solutions and also freezing techniques have been developed to
keep embryos alive for certain lengths of time outside of bodies. This opens up
the possibilities for flying embryos across oceans or across continents, rather than
animals themselves.
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Lesson 12

Another interesting application relates to endangered
species. Successful reproduction has already been
carried out by having common species carry and
mother the embryos from threatened species.
Embryos from threatened zebra species have been
"mothered" by horses, as an example.
Zebras (image by
MaleneThyssen)
Recombinant DNA
Genetic engineering, also referred to as Recombinant DNA Technology, involves
cutting a DNA segment (gene or genes) from one organism into small sections and
inserting (recombining) the sections in a '‘host' organism of the same or different
species. These transferred genes would then carry out their normal effects in the
new cells.
This action is not really new. In transformation, some bacterial cells can naturally
absorb bits of DNA from dead and disintegrated cells and incorporate them into their
own chromosomes. In transduction, viruses or bacteria can pick up and carry
pieces of DNA from one cell and into another.
Humans can now carry out genetic engineering. The DNA segments from the first
organism don’t automatically become part of the host
There are two types of
organism’s chromosomes. The sections are first attached
vectors. They are biological
to a ‘vehicle’ that will carry them into the cells of the host
and mechanical. Biological
organism. This vehicle or vector is a plasmid of a
vectors include plasmids
bacterium.
and viruses. Mechanical
A plasmid is a circular piece of DNA that is outside of the
chromosome of the bacteria. It carries different genes than
the larger chromosome. (see picture at bottom of page)
vectors are a micropipette
and a gene gun that shoots
a tiny metal bullet coated
with DNA into a cell.
Using a special restriction enzyme (bacterial proteins that have the ability to cut
both strands of the DNA molecule at a specific nucleotide sequence.), scientists can
remove a certain gene form human DNA. Another restriction enzyme is used to cut
the plasmid of the bacterium. The human DNA is then inserted into the opening in
the plasmid. Rejoining DNA fragments is referred to as gene splicing. A new
bacterium will take in the plasmid from the medium it is growing or living on. The
recombinant DNA will replicate along with the DNA of the bacterium.
Plasmids (2) are small rings of DNA. The large ring is the
Plasmids
bacteriums chromosome (1).
Image by Spaully
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Lesson 12
INSERT IMAGE HERE OF GENETIC ENGINEERING AND RECOMBINANT DNA STEPS
Gene "transplants" are also being increasingly performed between different species of
plants, animals and even between animals and plants. Certain human genes have
been inserted into the chromosomes of such varied organisms as cattle, pigs and
tomato plants.
The possibilities for practical applications of genetic engineering are vast. Some
examples are:

Pharmaceutical companies are making use of recombinant capabilities to ‘treat’
human diseases.

Recombinant bacteria are being used to produce phenylalanine. This is the amino
acid that is needed to make the artificial sweetener aspartame.
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Lesson 12

Genes responsible for human insulin production are being spliced into certain
bacteria. Recombinant bacteria produce large amounts of insulin. This is being
used to treat diabetes. Similar actions have been and can be carried out with
genes responsible for producing other hormones. Splicing such human genes into
bacteria or other organisms enables sufficient amounts of hormones to be
produced to treat various kinds of human defects.

In agriculture, farmers are hoping recombinant DNA can be used on bacteria in
the roots of legumes that would increase the rate of converting atmospheric
nitrogen into nitrates.

Genetic engineering has been used on bacteria that surround strawberries and
cause frost damage. After the ‘engineering’ has been done (the gene that causes
the frost damage is removed) the frost damage is prevented.

Genetic engineers are able to use mice, roundworms and the fruitfly, Drosophila
melanogaster as transgenic animals. They are able to do this because there are
many genes in common.

Genetic engineering has been used in plants so they resist herbicides, produce
internal pesticides and increase their protein production. It is more difficult to
engineer plants because of their cell walls and they do not have the plasmids that
bacteria have to take up the foreign DNA.
DNA technology has caused a “revolution” in bioctechnology – that is – relating the
use of living organisms to perform specific practical tasks. The manipulation of DNA
outside of living cells (in vitro) is more precise which makes it more distinct from
earlier work. Specific areas of work where DNA technology is revolutionizing are:
biological research, human medicine, criminal law, and agriculture.
Tools used to aid DNA Technology
Gel Electrophoresis
Gel electrophoresis refers to the technique in which
molecules are moved across a span of gel in a buffer
solution by an electric current. Molecules are separated on
the basis of size, electric charge, and other physical
properties.
Electro refers to the
energy of electricity.
Phoresis is from the
Greek verb phoros
which means “to
carry across.”
Gel electrophoresis is a valuable tool in genetic manipulation and study. Four of the
uses of gel electrophoresis are:
1.
Identification of specific DNA molecules by band patterns created in the gel
after they have been cut by restriction enzymes. Viral DNA, plasmid DNA and
some chromosomal DNA can be identified in this manner.
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Lesson 12
2.
Isolation and purification of specific fragments of DNA.
3.
Separation and identification of protein molecules right down to specific amino
acids.
4.
Determination of genetic differences and relationships between plants and
animals.
How and Why Gel Electrophoresis works.
How it works





The DNA samples are invisible so must be stained with a florescent stain called
Ethidium Bromide. (It glows pink under ultraviolet light).
A buffer solution is added to the gel. (Acts to maintain homeostasis)
Small samples of DNA are placed into wells that have been made at one end of
the gel.
Electrodes are connected – the negative post connected to the end where the
samples are placed and the positive end connected to a post at the end of the
gel, and the power is turned on.
The electricity is left to run for 15-20 minutes while the negatively charged
DNA travels towards the positive end. See the diagram for what a completed
gel would look like.
Why it works



The phosphates in DNA give the DNA molecule negative charge. This means that
DNA is soluble in water and can be attracted by a positive charge.
The DNA sample is put into a gel made from agarose (a polysaccharide extracted
from red algae) and water in a buffer solution (which maintains the proper pH and
salt concentration) to slow down the separating process so it doesn’t all happen at
once. A porous lattice forms from the agarose in the buffer solution. The DNA
pieces must slip through the holes in the lattice. The larger fragments will be
slowed down more than the smaller fragments, as it is easier for the smaller ones
to slip through the holes.
Smaller pieces of DNA move more quickly through the gel. The larger pieces
barely move at all. Thus, the DNA sample is separated by the size of its strands.
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Lesson 12
Gel Electrophoresis – separating DNA fragments
Restriction enzymes are the perfect tools for cutting DNA. However, once the DNA is
cut, a scientist needs to determine exactly what fragments have been formed. Once
DNA fragments have been separated on a gel, many other techniques, such as DNA
sequencing, can be used to specifically identify a DNA fragment.
The Process:
INSERT DIAGRAM SHOWING PROCESS OF GEL ELECTROPHORESIS
AND ALSO ON READING THE RESULTS
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Lesson 12
DNA fingerprinting
You have heard the phrase “dust for fingerprints” used by law enforcement officials at
a crime scene to hopefully identify who perpetrated the crime. Sometimes there are
fingerprints that can be identified and sometimes not.
DNA fingerprints can now be used to convict or acquit individuals suspected of a
crime. All that is needed is a small sample of DNA from the suspect. These samples
can include blood, hair, skin or semen.
Using the polymerase chain reaction (PCR) techniques the small samples can be
copied millions of times. A restriction enzyme is then used to cut the DNA into
fragments of different lengths. These fragments are separated by electrophoresis and
then compared to the DNA samples collected from the crime scene that have been
put through the same process.
Some implications of genetic engineering have created some grave concerns among
scientists and in the general public. Some of these concerns are:





There is a fear that potentially health threatening strains of bacteria could be
created which may escape laboratories and into the general public.
Characteristics of other organisms, plant and animal, may be altered to make
them (more) harmful to the environment or to humans.
Military powers could possibly create and unleash genetically recombined
forms that could have devastating results in the biosphere.
Public reluctance is also present to the consumption or use of plant and
animal products that have somehow been altered by insertions of human
genes.
Directly manipulating human reproduction by altering genes or chromosomes
is another major concern.
Questions will no doubt be asked more frequently as to what, or how far, genetic
engineering should be carried out, especially with respect to people.
Heredity and Environment
A question commonly asked concerns the importance or the respective roles played
by heredity and environment in the development of individuals. Could it be said that
one is more important than the other?
The actual chromosomes and genes (which form the genotypes) inherited from parent
cells or parent organisms form an extremely small mass in comparison to the final
mass of an individual adult organism. Almost all of this final mass is made up of
material that originates from soil, water and air. The actual genetic material or
genotype remains fairly stable throughout an individual's life. These genes tend to
control the way the substances making up bodies are put together to form the
phenotype. The phenotype is the sum total of all physical and chemical
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characteristics of a body. This includes such things as body size and form, color of
parts, types of enzymes or other chemical compounds and general composition. The
genotype directs the formation of nutrients into the distinct internal and external
features of an individual. This is what causes a certain seed to become a poplar tree,
a kitten to turn into a cat and what makes each one of us different from other people.
If we follow the development, growth and aging of any particular organism we would
most certainly see changes over time. The genotype or genes also "program" the
growth and aging process so that nutrients taken into bodies are put together and
function just a little bit differently than before. So while genotypes remain fairly
stable in an individual's lifetime, phenotypes are continually changing according to
genetic directions.
What has been mentioned so far could give the impression that it is the genotype
which is all-important. Yet, as livestock producers or plant growers know, the
environment can also have significant effects on individual phenotypes. Genotypes
can direct how nutrients are put together, but the availability or quantities of those
particular nutrients can determine sizes, shapes, colors or general health. Two
plants or two animals of similar genotypes, but grown or fed under very different
conditions or in different environments, can end up looking very different. Not only
nutrients, but sunlight, temperature, humidity or even the presence (crowding) of
other organisms could have major effects. Various diseases could also have major
effects on early developments or throughout organisms' lives.
At another level, substances from the environment could even affect gene
functioning. Radiation or chemicals could cause gene mutations. Even if genes are
not changed, substances could affect their functioning. As an example, some lower
organisms are formed with sex-influencing genes which produce female or male
characteristics. Ordinarily, these remain balanced to produce hermaphrodites.
However, certain environmental conditions (such as carbon-dioxide concentrations)
could change the gene behaviors to form either females or males.
To try and answer the question then, as to whether heredity or environment is more
important, is difficult. Both are important and both are continually interacting to
make organisms what they are, at particular times.
Summary
Understanding why organisms look (and function) as they do has been and is a
fascinating subject for many individuals. For the majority, it is simply a matter of
satisfying curiosity. For some, a knowledge of heredity and some of the principles
involved can play important roles in family planning or counselling, medical fields
and plant and animal production. While much information has already been learned
and accumulated, new discoveries and new applications should continue to make
this area of study interesting.
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Practice Questions
Dihybrid Cross
In pea plants:


tall stem length is dominant over short stem length
Yellow seed colour is dominant to green seed colour.
Answer the following questions based on the following cross:
A Homozygous tall, yellow seeded pea plant is crossed with a homozygous short green
seeded pea plant.
1.
What are the genotypes of the parents?
2.
What are the possible genotypes of the parent’s sex cells?
3.
What is the a) genotype and b) phenotype of the F1 offspring?
a)
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b)
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4.
What are the possible genotypes of the sex cells of the F1 parents?
5.
Work out the possible genotypes of the F2 generation in the Punnett Square.
6.
How many phenotypes are present in the F2 generation? ____________________
What are these?
__________________, __________________, __________________, __________________
7.
What ratio would you expect for the phenotypes? (from Punnett Square results)
________________________________________
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Practice Genetic Questions
1.
In the land of Zigs, green colour (G) is dominant to red colour (g). If a
homozygous green zig marries a homozygous red zig, what will the F1
generation be comprised of? If two of these married (hybrids) what are the
likely results? Give phenotypes and genotypes of the offspring as well as the
phenotypic ratio.
2.
Two parents are each heterozygous for the recessive gene of blue-eyed.
Discuss the probability, that if they have four children, of the number of
brown-eyed and blue-eyed children that will result..
3.
A strain of naked mole rats (voles) has uniquely short ears and this is inherited
as a recessive character. If a homozygous brown (BB), short-eared (rr) vole is
mated with a white (bb), regularly eared (RR) vole what types of offspring are
expected in the F1? Give all the possible gametes from the F1 parents.
4.
In horses black (B) is dominant to chestnut (c) and trotting (T) a gait in which
the legs move in pair’s diagonally but not quite simultaneously is dominant to
pacing (p) in which the legs move in lateral pairs. A black pacer is bred to a
chestnut trotter and the resulting colt is a chestnut pacer. Give the genotypes
of the parents and offspring.
5.
A red flowered petunia plant was crossed with one that had white flowers.
a.
b.
c.
d.
What is the phenotype of the F1?
Of the F2?
Phenotype ratio of the offspring of a cross of the F1 back to its red
parent?
Phenotype ratio of the offspring of a cross of the F1 back to its white
parent?
(Note: In petunia flowers, red color is incomplete dominate over white, the
heterozygous plants being pink.)
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6.
If you wanted to produce petunia seed all of which would yield pink-flowered
plants when sown, how would you do it?
7.
In the breed of Schnauzer dogs, the gene for normal behaviour (N) is dominant
over the gene for shy behaviour (n), if a dog homozygous for normal behaviour
married one which is shy, how would their offspring act? Give the phenotype
and genotype of the offspring.
8.
A blue-eyed man both of whose parents were brown-eyed marries a brown-eyed
woman whose father was brown-eyed and whose mother was blue-eyed. They
have one child, who is blue-eyed. What are the genotypes of all of the
individuals mentioned?
9.
The gene (R) for rhumba is dominant over that (r) for waltzing in kangaroos. If
a kangaroo, homozygous for rhumba married one which is homozygous for
waltzing, what would their favourite dance be? Give the phenotype and
genotype of the offspring. Diagram (use Punnett Square) the F1 cross if two of
the offspring were mated. Give the genotypic and phenotypic ratios.
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Dihybrid Cross Answer Sheet
Garden Peas: a homozygous tall, stemmed yellow seed colour plant × homozygous
short stemmed, green seed colour plant.
T – Tall Stem is dominant
t – short stem is recessive
Y – yellow seed is dominant
y – green seed is recessive
1.
TTYY × ttyy
2.
TY
3.
a.
b.
TtYy
Tall stemmed Yellow seed colour
4.
a.
b
c.
d.
TY
Ty
tY
ty
ty
5.
6.
4. Tall yellow, tall green, short yellow, short green
7.
9:3:3:1
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Answers to Practice Questions
1.
Parent cross is : homozygous green × homozygous red
(GG × gg). The F1 is Gg genotype and the phenotype is all green.
The results of an F1 cross are:
Gg × Gg
genotype ratio = 1GG : 2Gg : 1gg
phenotype ratio = 3 green zigs : 1 red zig
2.
B = brown eyed, b = blue eyed
Parent Cross: Bb × Bb
genotype ratio = 1BB : 2Bb : 1bb
phenotype ratio = 3 brown eyed, 1 blue eyed
3.
Parent Cross: BBrr × bbRR
All of the F1 are BbRr
Gametes = BR, Br, bR, br
4.
Colt = ccpp (chestnut pacer). The colt received one c from each parent and one p from each
parent so the genotypes of the parents black pacer × chestnut trotter is:
Bbpp × ccTt
Bp,bp
cT,ct
5.
Cross: red flowered × white flowers
RR × WW
a.
phenotype of F1 = RW
b.
phenotype of F2 = 3 red : 1 white
c.
phenotype ratio of cross of F1 back to
red parent (cross: RW × RR) is 1 red : 1 pink
d.
phenotype ratio of cross of F1 back
to the white parent (cross: RW × WW)
is 1 pink : 1 white
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6.
You would not be able to produce seed that all yielded pink-flowered plants. The solid red and
white would always occur.
7.
Their offspring would act normal.
cross: NN × nn
Nn
This is the phenotype. The genotype is Nn.
8.
Blue-eyed man has genotype of bb. His parents are both Bb.
The woman’s father definitely has one B, the other gene we have to look at the daughter who is
the woman in this question. Her mother is bb. Look at the woman to determine the second
gene of the father. The woman is brown-eyed but we know her mother was bb so the woman
has to have one b so the woman is Bb so her father can be either BB or Bb (likely BB). The
child is bb.
9.
Their favorite dance would be the rhumba.
cross: RR × rr
genotype of F1: Rr
phenotype of F1: rhumba
F1 cross: Rr × RR
genotypic ratio: 1RR : 2Rr : 1rr
Another way of saying this is 1 homozygous for rhumba : 2 heterozygous for rhumba : 1
homozygous for waltzing
phenotypic ratio: 3 Rhumba : 1 waltz
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