Download Genetics slide 8

Fundamentals of
Genetics
Gregor Mendel
Gregor Mendel
was a monk in mid
1800’s who
discovered how
genes were passed
on.
 He used peas to
determine the
pattern of heredity

7 Characteristics of
Pea Plants
1. height of plant (tall and short stem)
 2. color of pea pods (green or yellow)
 3. pod shape (inflated or wavy)
 4. feel of peas (smooth or wrinkle)
 5. color of peas (green or yellow)
 6. flower positions (on sides or end of
stem)
 7. flower colors (purple or white)

7 Characteristics of Pea
Plants
Mendel noticed
that the plants
carried different
characteristics, or
traits
 He further noticed
that there were two
strains for each
trait.
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Mendel had to pollinate
pollination:
the peas
combining pollen
from the male part of
the plant to the
female part of the
plant (i.e., mating the
plant)
pollen comes from
the anthers: male
reproductive organ of
plants. Stigma:
female reproductive
organ of plants
Obtaining the Pure
strain
 self-pollination:
mating male &
female of the same plant or flower
 cross-pollination: mating from
flowers of 2 different plants
 By using these techniques, Mendel
achieved plants that were pure for
each given strain
Mendel’s Experiments
 The
pure plants for each strain were
called the Parent or P1 generation
 The offspring of the P1 generation
were called the first filial, or F1
generation
 The offspring of the F1 generation,
the second filial, were called the F2
generation. These would be the
“grandchildren” of the P1 generation
Mendel’s Results and
Conclusions

In one of his
experiments,
Mendel crossed a
plant pure for
green pods with a
plant pure for
yellow pods
Mendel’s Results and
Conclusions
 To
his surprise,
all of the
offspring, or F1
generation, had
green pods
Mendel’s Results and
Conclusions

However, when he
crossed plants from
the F1 generation, the
resulting F2
generation had about
three-fourths green
pods and one-fourth
yellow pods
Mendel’s Results and
Conclusions
Mendel concluded that something within
the pea plants controlled characteristics,
which he called factors
 Since each characteristic had two forms,
he concluded their must be a pair of
factors controlling each trait

Mendel’s Results and
Conclusions
Whenever Mendel crossed strains, one of
the P1 strains failed to appear in the F1
plants.
 However, in every case, that trait
reappeared in a ratio of about 3:1 in the
F2 generation

Mendel’s Results and
Conclusions

Mendel reasoned that
the trait appearing in
the F2 generation was
controlled by a
dominant factor
because it masked, or
dominated the other
factor
Mendel’s Results and
Conclusions
Since the trait reappeared in the F2
generation, it must be controlled by a
recessive factor
 Thus, a trait controlled by a recessive
factor had no observable effect on an
organism’s appearance when it was
paired with a trait controlled by a
dominant factor

Law of Segregation



The paired factors separate during the formation
of sex cells.
Each reproductive cell receives only one factor
from each cell
When the gametes combine during fertilization,
the organism will again have two factors
controlling each trait
Law of Independent
Assortment


Mendel also found that
no relationship existed
between different
characteristics.
Thus, factors for
different characteristics
are distributed to
gametes independently
Chromosomes and
Genes
Mendel’s studies agree with modern
molecular genetics.
 A gene is a segment of a chromosome that
controls a particular trait
 Since chromosomes occur in pair, genes
also occur in pairs
 What Mendel called “factors” are now
known as alleles

Chromosomes and
Genes




Letters are used to
represent alleles
Capital letters refer to
dominant alleles
Lowercase letters
refer to recessive
alleles
The letter used is not
important
Chromosomes and
Genes

Each trait would be
represented by a pair
of letters, each
representing an allele
from a parent
Checking For
Understanding





Heredity: the genes that are passed on to the
offspring
Pure bred: plants that will always produce the
same offspring
Genetics is the study of how genes are passed
from parents to offspring.
Strain (breed): plants that has a specific or
unique trait
P (or P1) generation: the parent generation (the
first mating)
Checking For
Understanding
F1 generation: the first offspring
generation (children)
 F2 generation: the second offspring
generation (grandchildren)
 self-pollination: mating male & female of
the same plant or flower
 cross-pollination: mating from flowers of
2 different plants

Checking For
Understanding

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Dominant genes: those genes that are expressed
and seen
Recessive genes: genes that are shaded over and
do not show
Every trait that you have must have a dominant
and/or recessive gene
The two genes do not separate during mitosis
Dominant and recessive genes only separate
during meiosis or making gametes
9.2 Genetic Crosses
Objectives




Explain how probability is used to predict the
results of genetic crosses
Use a Punnett square to predict the results of
monohybrid and dihybrid crosses
Explain how a testcross is used to show the
genotype of an individual whose phenotype is
dominant
Differentiate a monohybrid cross from a
dihybrid cross
Genotype and
Phenotype
Phenotype is what the organism looks
like, that is, its outward appearance
 Genotype is the genetic makeup of an
organism

– For example, an organism could show the
phenotype for brown eyes, yet have either the
BB or Bb genotype
Homozygous
Heterozygous

An organism is homozygous for a trait if
the genotype is either pure dominant (e.g.,
BB) or pure recessive (e.g., bb)
– Thus, BB would be called homozygous
dominant, and bb would be referred to as
homozygous recessive

An organism is heterozygous for a trait if
the allele is mixed, as in Bb
Probability

Probability tells what to expect
Probability =
Number of times an event is expected to happen
Number of opportunities for the event to happen
Predicting Results of
Monohybrid Crosses


A cross between two
individuals that only
involves one trait is
called a monohybrid
cross
We use what is called
a Punnett square to
predict the results
Punnett Squares
First determine the genotypes for the two
parents (the P1 generation)
 For this demonstration we will use T for
tall (the dominant allele) and t for short
(the recessive allele).
 Mother is short (homozygous recessive) so
we will use tt
 Father is tall (heterozygous), so we will
use Tt

Start by drawing a box
with four squares
Take the father’s
genotype and split the
letters on the side
Take the
mother’s
genotype and
split the letters
on the top
Copy the letters down
and across to fill in the
first box
And the next box…
And the next box…
And the last box!
Determining Phenotype
Ratios
We see that we have



50% tall and 50%
short, or 2 out of 4 for
each.
We could write this
as 2:2, using the
dominant trait first.
With phenotypes, we
can also use
percentages, such as
50:50
Determining Genotype
Ratios


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We always write
genotype ratios in the
following format:
DD:Dd:dd, or
homozygous dominant:
heterozygous:
homozygous recessive.
In this square we have a
ratio of 0:2:2
Always include the zero
Example 1
Homozygous x Homozygous



This cross shows a
cross between a plant
homozygous
dominant for purple
flowers with a plant
homozygous
recessive for white
flowers
GRATIO = 0:4:0
PRATIO = 4:0
Example 2
Homozygous x Heterozygous



This cross shows a
cross between a
guinea pig
homozygous
dominant for black
coat with one
heterozygous
GRATIO = 2:2:0
PRATIO = 4:0
Example 3
Heterozygous x Heterozygous



Here both rabbits
show the dominant
black coat, even
though they are
heterozygous for the
trait. Notice how the
offspring shows the
recessive trait
GRATIO = 1:2:1
PRATIO = 3:1
Example 4
Test Crosses
Sometimes you have an organism with a
recognizable phenotype, such as curly hair
(A). However, you cannot tell if the organism
is homozygous dominant (AA) or
heterozygous (Aa)
A test cross can help you determine the
organism’s genotype as well
Cross the organism with another organism
homozygous recessive (aa) for that trait
AA x aa

Genotype ratio 0:4:0
Phenotype ratio 4:0
If the organism is
AA, a cross with aa
will give all offspring
showing the dominant
trait, regardless of
how many offspring
are generated
Aa x aa

Genotype ratio 0:2:2
Phenotype ratio 2:2
If, however, about
half of the offspring
show the recessive
trait, then you can be
certain that the
genotype of the test
organism was
heterozygous
Example 5
Incomplete Dominance


Sometimes neither
allele is completely
dominant over the
other
This results in the Rr
showing both factors
in its phenotype
Example 6
Codominance

•The coat of this cow
consists of both red
and white hairs. Both
the red and white
phenotypes show

Codominance occurs
when both alleles for
a gene are expressed
in a heterozygous
offspring
In codominance,
neither allele is
dominant, nor do the
alleles blend as in
incomplete
dominance
Dihybrid Crosses
For this cross both parents will be
heterozygous for round yellow (RrYy) peas

Place all four alleles for one parent along the top,
and those for the other parent along the side
Dihybrid Cross


Combine the parent
alleles to create the
possible combinations
for the offspring.
Remember!! Each
offspring must have 4
letters (two alleles for
each trait)
Checking for
Understanding
The genotype is the genetic makeup (e.g.,
DD, Dd, dd) of an organism
 The phenotype is the outward appearance
(brown hair, blue eyes)
 Probability is the likelihood that an event
will take place
 A Punnett square can be used to predict
outcomes of genetic crosses

Checking for
Understanding
A cross involving only one trait (one pair
of alleles) is a monohybrid cross
 A cross involving two traits (two pairs of
alleles) is a dihybrid cross
 A testcross can determine if an organism
is DD or Dd
 Complete dominance occurs when DD
and Dd have the same phenotype

Checking for
Understanding
Incomplete dominance occurs when the
phenotype shows a blend of both traits
 Codominance occurs when both traits
show without blending in the phenotype
