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Biology
HSC Course
Stage 6
Blueprint of life
Part 2: Patterns of inheritance
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Contents
Introduction ................................................................................2
Gregor Mendel and his work ......................................................4
The principles of heredity .........................................................15
Monohybrid crosses ...........................................................................15
Mr Punnett and his squares...............................................................19
Investigating human variation ..................................................22
Pedigrees.................................................................................25
Constructing a pedigree.....................................................................25
Using a pedigree to investigate a trait ...............................................28
Using a pedigree to make predictions ...............................................30
Hybridisation ............................................................................33
Suggested answers .................................................................35
Exercises – Part 2....................................................................41
Part 2: Patterns of inheritance
1
Introduction
The idea that offspring inherit features from their parents has been known for a long time. It
was believed that the features inherited from each parent blended together.
The Austrian monk, Gregor Mendel, decided to breed pea plants and study one clearly
defined characteristic at a time. He kept a careful statistical record of results and his
conclusions became the basis of the laws used to explain how offspring inherit features from
their parents.
Once fundamental patterns of inheritance were understood the ability to predict the types of
offspring likely to be produced became possible. This study of inheritance gave birth to the
science of genetics and was a boost to agriculture and breeding. Today it is important to
medical practitioners investigating the likelihood of people inheriting diseases.
This section outlines the experiments carried out by Mendel, now regarded as the founder of
genetics, and explains how his experimental techniques led to an understanding of the patterns
in inheritance. This will enable you to describe the basis of Mendel’s experiments and
teaches you skills that you can use to solve problems involving inheritance.
In this part you will have opportunities to learn to:
•
outline the experiments carried out by Gregor Mendel
•
describe the aspects of the experimental techniques used by Mendel that led to his
success
•
describe outcomes of monohybrid crosses involving simple dominance using Mendel’s
explanations
•
distinguish between homozygous and heterozygous genotypes in monohybrid crosses
•
distinguish between the terms allele and gene, using examples
•
explain the relationship between dominant and recessive alleles and phenotype using
examples
•
outline the reasons why the importance of Mendel’s work was not recognised until some
time after it was published.
In this part you will have the opportunity to:
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•
perform an investigation to construct pedigrees or family trees, trace the inheritance of
selected characteristics and discuss their current use
•
solve problems involving monohybrid crosses using Punnett squares or other appropriate
techniques
•
process information from secondary sources describe an example of
hybridisation
within
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of life
a species and explain the purpose of this hybridisation.
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Extract from Biology Stage 6 Syllabus © Board of Studies NSW, originally issued 1999. The most
up-to-date version can be found on the Board's website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_lista.html
This version October 2002.
Part 2: Patterns of inheritance
3
Gregor Mendel and
his work
It requires indeed some courage to undertake a labour of such far-reaching extent; this
appears, however, to be the only right way by which we can finally reach the solution of a
question the importance of which cannot be overestimated in connection with the history of
the evolution of organic forms.
From Mendel’s famous paper, Experiments in Plant Hybridisation, published in 1865.
In the mid 1800s, an Austrian monk and keen botanist called Gregor Mendel observed that the
offspring of certain plants had physical characteristics similar to those of its parents and
ancestors. He wondered why related organisms had a tendency to resemble one another and if
it could be explained.
Observing traits
Mendel reasoned that close observations of inheritance might provide the answers. So in
1857 he decided to conduct breeding experiments with the pea plant (Pisum sativum) because
it had a short reproductive cycle. He selected plants and caused them to reproduce. This is
called performing a cross (X), or crossing the plants. Then he compared features of the parent
plants with those of the new plants produced.
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Mendel studied seven separate characteristic features of pea plants. These features or
characteristics are referred to as character traits (pronounced trays with a silent ‘t’ near the
end). The seven traits Mendel examined were plant height, flower colour and position, seed
colour, seed shape and pod colour and shape.
He noted that each trait comes in two different forms or factors. For example, the seed pod
colour is either yellow or green. This means that the trait for pod colour has a yellow factor
or green factor.
Examinations of the seed shape factor led to an interesting observation. Mendel wrote:
the hybrid character resembles that of one of the parental forms so closely that the other
either escapes observation completely or cannot be detected with certainty.
A hybrid refers to the offspring that result from a cross between parents with different factors.
When Mendel used plants with round peas and ones with wrinkled peas as
the parents, only plants with round peas were observed.
1
Which plants were crossed?
_____________________________________________________
_____________________________________________________
2
What feature do the offspring (or progeny) have?
_____________________________________________________
3
What is the hybrid character that Mendel refers to in the extract above?
_____________________________________________________
Check your answers.
Dominant and recessive traits
Mendel stated that those characters that are transmitted unchanged are termed dominant. The
feature not visible in the appearance of the offspring was said to have ‘hidden’ or ‘stored’
hereditary information.
In Mendel’s experiment, when the round pea hybrids were crossed, the wrinkled factor
reappeared unchanged in some of their offspring. Mendel wrote:
the expression ‘recessive’ has been chosen because the characters thereby designated
withdraw or entirely disappear in the hybrids, but nevertheless reappear unchanged in their
progeny.
Each of the seven traits that Mendel studied had a dominant factor and a recessive factor. The
feature exhibited by all of the offspring in the first generation is the dominant factor and the
recessive factor is the feature that has been hidden but is shown, or expressed again in the
second generation.
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Mendel’s results are summarised in the following table.
Expt
Trait examined
First generation
Second generation
1
seed shape
all round
5474 round,
1850 wrinkled
2
seed colour
all yellow
6022 yellow,
2001 green
3
flower colour
all violet-red flowers
705 violet-red,
224 white
4
form of seed pods
all inflated pods
882 inflated,
299 constricted
5
colour of seed pods
all green pods
428 green pods,
152 yellow
6
position of flower
all axial
651 flowers axial,
207 flowers terminal
7
length of stem
all long stem
787 long stem,
277 short stem
Use information from the previous table to complete these tasks.
1
Has the recessive factor been destroyed in the first crossing?
How did Mendel explain its reappearance?
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
Complete the table below. The first row has been done for you.
Expt
Trait examined
Dominant
factor
Recessive
factor
Ratio in second
generation
1
seed shape
round
wrinkled
5474
round
=
wrinkled 1850
2
seed colour
3
flower colour
4
form of seed pods
5
colour of seed pods
6
position of flower
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length of stem
3
Are the ratios for the second generation similar? ______________
4
What is the average ratio of dominant to recessive factors in the second generation of all
the traits examined?
_____________________________________________________
5
In terms of scientific method, why did Mendel use such a large number of pea plants for
his investigations?
_____________________________________________________
_____________________________________________________
Check your answers before reading on.
Mendel’s conclusions
From his investigations with pea plants, Mendel made some conclusions about how
characteristics are inherited.
•
Each characteristic is controlled by a pair of factors in the parents which are passed on in
the gametes (sex cells that form offspring).
•
These factors separate when gametes are formed. The gametes then contain only one
factor from the pair.
•
At fertilisation, gametes unite at random therefore ratios of offspring are predictable.
•
One factor (the dominant factor) may mask the effect of the other (the recessive factor)
when both are present.
•
The factors are unchanged from generation to generation, even when their effect is not
expressed (does not produce an observable feature).
Mendel’s success
Mendel succeeded in finding a pattern to explain and predict the inheritance of many features.
From what you have read about his experiments, why do you think he was successful in
finding the pattern?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Here are some features of his experimental techniques that contributed to his success.
Part 2: Patterns
• of inheritance
Mendel studied
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character traits that had a clearly distinguishable pair of factors.
•
Mendel carried out an enormous number of experiments using a large number of plants
for each.
•
Mendel took care to keep the plants of one experiment isolated from other experiments.
He personally transferred the pollen from one plant to the other using hand pollination
techniques. He also collected the seeds and sowed them himself.
•
Mendel rigorously collected data and kept a thorough record of results obtained. He used
mathematical and statistical analysis on this data to interpret the results of his breeding
experiments.
Explaining Mendel’s observations
Mendel explained his observations by denoting the dominant factor with a capital letter (eg.
A) and the recessive factor with a small letter (eg. a). Each individual has a pair of factors.
Therefore, the possible factors that individuals can have for this feature are:
•
AA pure dominant form
•
aa pure recessive form
•
Aa hybrid form.
Mendel used letters such as these to explain how the 3:1 ratio was obtained in each of his
experiments. Remember that offspring inherit one factor for the feature from each parent
during crossbreeding.
Looking at an example
Consider the example of round seeds (R) and wrinkled seeds (r).
Complete the following table for this feature.
Form
Factors
pure dominant
pure recessive
hybrid
Check your answers.
What will happen if a pure dominant (round-seeded) parent is crossed with a pure recessive
(wrinkle-seeded) parent?
The pure dominant parent can only produce
gametes that contain the dominant (R) factor.
The diagram to the right shows how this is
often represented.
RR
R
parent
R gametes
The pure recessive parent can only produce gametes with the recessive (r) factor.
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Because each offspring must inherit a factor from each parent, you can see that the progeny in
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the first generation can only ever be Rr.
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Here is a diagram to represent this cross.
wrinkled (rr)
round (RR)
X
RR
Rr
rr
Rr
Rr
Rr
All round but carry the ‘hidden’ factor for wrinkled peas (Rr)
So all the offspring of a cross between pure dominant round-seeded plants (RR) and pure
recessive wrinkle-seeded plants (rr) are
round-seeded hybrids (Rr).
Now look at what happens when you cross the hybrid forms to produce the second generation.
round (Rr)
round (Rr)
X
Rr
RR
Rr
Rr
Rr
rr
round : wrinkled = 3 : 1
Did you notice how each hybrid parent can contribute a dominant (R) factor or a recessive (r)
factor? To determine the possible combinations of these factors, Mendel used a mathematical
device called binomial expansion. Here is an explanation.
There are four possible ways the offspring can inherit a factor from each parent.
Part 2: Patterns of inheritance
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Using the first factor
in each pair:
Rr
X
Rr
RR
Using the second factor
in each pair:
Rr
X
Rr
rr
Using the inside factors
in each pair:
Rr
X
Rr
Rr
Using the outside factors
in each pair:
Rr
X
Rr
Rr
There is an equal chance of each of these four possibilities. Therefore, if you observe a large
enough number of crosses (and Mendel used thousands), there should be an equal proportion
of each.
Hence:
•
one quarter will be RR, pure round-seeded
•
one quarter will be rr, pure wrinkle-seeded
•
two quarters (or a half) will be Rr, the hybrid round-seeded form.
The ratio predicted by this explanation is –
round-seeded plants : wrinkle-seeded plants = 3 : 1
This was the ratio that Mendel observed.
Another example
The diagram below represents the observed results of Mendel’s experiments to study the
inheritance of plant stem length in pea plants.
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Parents
(one pure tall,
one pure short)
X
First generation
(all tall)
X
Second generation
(787 tall and 277 short)
Using the letter T for the dominant factor and t for the recessive factor, answer the questions
that follow.
1
Mendel used pure long-stemmed plants and crossed them with
short-stemmed plants. What factors were inherited in the
first generation of plants?
_____________________________________________________
2
What ratio of long-stemmed to short-stemmed plants was observed in the second
generation?
_____________________________________________________
3
Use letters to represent the possible offspring in the second generation.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 2: Patterns of inheritance
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_____________________________________________________
Check your answers.
Recognition of Mendel’s work
After eight years of collecting data from nearly 30 000 pea plants, Mendel discovered the
natural laws of inheritance. Mendel’s findings on plant hybridisation were presented in two
lectures before the Society for the Natural Sciences in 1865 in Brünn, Moravia (now called
Brno in the Czech Republic).
His paper, Versuche über Pfanzenhybriden, was published in the Society’s proceedings in
1866 and sent to 133 other associations of natural scientists and to the more important
libraries in a number of different countries.
Unfortunately, there was no response to his findings and its importance was not recognised
until 34 years after the publication of his results.
Mendel’s approach and the nature of his experimentation were simply too unconventional for
his age. Nobody before him had ever attempted to use mathematical and statistical analysis as
a means of interpreting the results of biological experiments. Biologists in those days were
mostly people interested in natural history (observing and recording the features and
behaviour of things in the world around them) and not necessarily familiar with mathematics.
Mendel sent his reports to the most famous botanist in Europe at the time, Karl Wilhelm von
Nageli of Switzerland. He hoped to gain his sponsorship (support of his work) but Von
Nageli ignored Mendel’s work and sent it back to him. Mendel was able to get his paper
published in a scientific journal several years later but no-one acknowledged it because he
was an unsponsored amateur.
Another reason for the absence of any response from the scientific fraternity of the day was
the limited number of people who read the Brno Association’s records. Also Mendel was
known as a relatively shy person and might not have presented his results with the necessary
emphasis and stress to get others to notice them.
Just before his death on January 6, 1884, he commented:
my scientific labours have brought me a great deal of satisfaction, and I am convinced that
before long the entire world will praise the result of these labours.
And that is what happened. By the end of the nineteenth century, breeders and horticulturists
were looking for a reliable set of rules to ensure the propagation of certain features in their
crops and livestock.
Then, in the spring of 1900, an amazing coincidence occurred. Three botanists, Hugo de
Vries from Holland, Carl Correns from Germany and E. von Tschermak from Austria,
independently reported the same conclusions as Mendel. This amounted to a rediscovery of
Mendel’s inheritance research, verifying his work and giving rise to a new branch of biology
called genetics. Each of these men gave full credit to Mendel for the discovery.
Now complete Exercise 2.1.
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The principles of
heredity
You have been studying Mendel’s experiments that examine the inheritance of a single trait
involving one pair of factors. Breeding experiments of this kind are called monohybrid crosses.
(Mono- refers to the one feature being studied and -hybrid refers to the two forms of the feature.)
Monohybrid crosses
Monohybrid crosses are the simplest way to study how features are inherited. There are only
four kinds of monohybrid crosses involving simple dominance. They are:
•
crosses of two of the same pure breeding forms
For example, AA AA or aa aa
The offspring of these crosses will always be the same as their parents
•
crosses of a pure breeding dominant with a pure breeding recessive
For example, AA aa
You will recall that this type of cross results in all the offspring having the hybrid form of
the feature. In this case, Aa.
•
crosses of a pure breeding dominant with the hybrid form
For example, AA Aa. If you use the technique explained previously you will notice the
following outcome:
AA
AA
Aa
Aa
AA
Aa
Half of the offspring are pure breeding dominant and the other half are hybrids.
However, they will all look alike.
•
crossing a pure breeding recessive with the hybrid form
For example, aa Aa. Use the technique above to determine the
possible outcomes of this cross. Show your working as a diagram.
Check your
Part 2: Patterns of inheritance
answer.
13
Mendel’s explanations were based on the language of his day. It has limitations when trying
to describe the complexities of today’s genetics. You need to learn how to use some new
words so that you can go on further with this module.
Homozygous and heterozygous
An organism with a pure breeding trait is said to be homozygous for that feature. So AA and
aa are both homozygous; AA is pure breeding dominant and aa is pure breeding recessive.
The hybrid character is called heterozygous. Aa is an example.
To help you to remember these terms better, just remember the word stem:
•
homozygous – when a trait has the same two factors
•
heterozygous – when a trait has two different factors.
To help you to recognise and use these terms better, complete the following
tasks then check your answers.
1
Brown eyes in humans are dominant over blue eyes. Using the letters
B for dominant and b for recessive, write the letters for:
a) homozygous for blue eyes ____________________________
b) homozygous for brown eyes ___________________________
c) heterozygous for brown eyes __________________________
2
Predict the likelihood of a couple, heterozygous for brown eyes,
having a baby with blue eyes.
_____________________________________________________
_____________________________________________________
Check your answers.
Genes and alleles
The word ‘factor’ is not used much today because it is now known that genes are responsible for
transmission of inherited information. The word gene comes from the Greek word genesis that
means birth. The term genetics was coined by the famous British biologist William Bateson in
1902.
A gene is a unit of genetic material with information about one feature. Alleles are alternative
forms of a gene. For example, the height of a pea plant (length of stem) depends on the
organism having genes for tallness. Each plant has two genes for this trait. The plant can be
pure breeding tall (homozygous, TT) with identical alleles; it can be hybrid tall (Tt) with
different alleles; or it can be pure breeding short (tt) with identical alleles.
Consider the cross below.
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hybrid tall plants
X
A
C
B
D
E
F
Which plant(s) definitely:
1
has/have identical alleles? ________________________________
2
has/have different alleles? ________________________________
3
is/are homozygous? _____________________________________
4
is/are heterozygous? ____________________________________
Check your answers.
Complete Exercise 2.2 now.
Genotype and phenotype
In Mendel’s experiments, each trait observed was a result of the expression of one or both
genes in a pair of genes. (Not only are genes responsible for the transmission of information
but they control what an organism looks like.)
The appearance of a trait in an organism is called its phenotype. The genetic information that
produces the trait in the organism is called its genotype. For example, for the feature of plant
height (length of stem), the ‘tallness’ phenotype can be expressed by two genotypes – TT or
Tt. The genotype for the recessive short phenotype is tt.
Whenever you do problems and calculations involving genotype, use the letters that represent
the genes. When describing phenotype, just describe the looks of the trait in the organism.
1
Complete the table below for the trait of seed shape, that involves the
alleles R (the gene for round seeds) and r (the gene for wrinkled seeds).
Part 2: Patterns of inheritance
Phenotype
Genotype(s)
15
round seeds
wrinkled seeds
2
Why are there only two phenotypes for this trait?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Mr Punnett and his squares
Reginald Crundall Punnett was born in England in 1875. He was a geneticist who, with
William Bateson, discovered genetic linkage. (You’ll learn more about genetic linkage later
in the module.)
Punnett’s work helped to establish Mendel’s theories. In fact, in 1905 he wrote a book called
Mendelism.
In 1912, Punnett became a fellow of the Royal Society of London and was named professor
of genetics at Cambridge. He is best known for devising the Punnett square for summarising
the fusion of gametes in genetic crosses. (Punnett is spelt in a variety of ways in different
books, including Punnet, punnett and punnet.)
A Punnett square is constructed in the following way.
1
Draw a 3 3 grid.
+
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2
Write the alleles for one parent
(often the male) in the left
column and the alleles for the
other parent (often the female)
in the top row.
For example, Rr Rr
R
r
R
r
R
RR
Rr
r
Rr
rr
+
R
r
3
Fill in the empty squares by
combining the gametes as
shown. When you combine
the genes in the gametes, you
are finding out the genes that
are in the offspring.
+
So the offspring have genotypes of RR, Rr, Rr and rr. That is, one quarter are homozygous
dominant, one half are heterozygous and one quarter are homozygous recessive. Their
phenotypes for this trait will be three with the dominant feature to one with the recessive
feature, or 3 : 1.
Use Punnett squares to solve the following problems.
1
What are the possible genotypes and phenotypes for the offspring of a
couple who have brown eyes? The male is homozygous (BB) and the
female is heterozygous (Bb).
a) Draw a 3 3 grid.
(It has been done for you.)
b) Write the alleles for one
parent in the left column
and the alleles for the other
parent in the top row.
+
c) Fill in the empty squares by
combining the gametes.
d) Now answer the question in the problem.
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
2
What is the genotype and phenotype of a person who has a homozygous brown-eyed
mother and a blue-eyed father?
a) Draw and complete a Punnett square.
Part 2: Patterns of inheritance
17
b) Answer the question in the problem.
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
3
Determine the possible genotypes and phenotypes for the offspring of a couple where the
male is blue-eyed (bb) and the female is brown-eyed but has a blue-eyed father.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
In each of the problems above, the characteristic (eye colour) can have two phenotypes but
there are three possible genotypes (BB, Bb and bb).
4
Using this example, explain the relationship between dominant and recessive genes and
the phenotype observed.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
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_____________________________________________________
Check all your answers.
Complete Exercise 2.3 now.
Part 2: Patterns of inheritance
19
Investigating
human variation
You have been looking at the variation that Mendel studied in pea plants. Variation in
humans has also been extensively studied. Complete your own investigation of human
variation by reading and following the instructions below.
Aim
A single pair of genes (alleles) controls certain traits or characteristics in
humans. The aim of this activity is for you to examine some of these human
inherited traits in yourself and someone you know.
Procedure
Read the list of some human traits and their descriptions below. Then determine which of
these characteristics you possess. Record your phenotype and genotype in the result table on
the next page. Then identify and record the information for a friend or family member.
List of some human traits
20
•
ear lobes
may be attached or free. Free ear lobe (G) is dominant over
attached (g)
•
cheek dimples
may be present in one cheek only, in both cheeks or there may
be more than one dimple in each cheek. Dimples (D) is
dominant over no dimples (d)
•
tooth gap
may occur in the centre of the top row of teeth. Frontteeth
close together (T) is dominant over a tooth gap (t).
•
vision
short or long sight (V) are dominant over normal vision (v)
•
eyelashes
long eyelashes (S), 10 mm or more, are dominant over short
eyelashes, less than 10 mm (s). (Use a small ruler to measure
the length of eyelashes)
•
tongue rolling
the ability to roll the sides of the tongue upwards to form a
U-shape. This ability is dominant (R) over inability to roll the
tongue (r)
•
widow's peak
where the hairline on the forehead comes down to a peak in the
middle is dominant (W) over a straight or curved hair line (w)
•
freckles
spots on the skin (freckles) are dominant (F) to no freckles ((f)
•
mid digital hair
hair on the back of the middle segments of the fingers. Hair on
the middle segment of one or more fingers (H) is dominant over
lack of hair on the middle segments of the fingers (h)
•
handedness
of life
right-handedness (P) is dominant over left-handedness Blueprint
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•
second toe
length
the second toe may be shorter than the big toe, which is
dominant (B), or longer than the big toe, (b)
•
hair form
determined by a pair of genes, which are incompletely
dominant. Homozygous gene combinations produce straight
hair (CC) or curly hair (cc) while the heterozygous combination
produces wavy hair (Cc). You’ll learn more about this in Part 3.
Results
Characteristic
For you
Phenotype
Possible
genotype
For a friend or relative
Phenotype
Possible
genotype
ear lobes
cheek dimples
tooth gap
vision
eyelashes
tongue rolling
widow's peak
freckles
mid digital hair
handedness
second toe length
hair form
Discussion
Compare your features with someone else in your family. Can you see any pattern in features
that may have been inherited?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 2: Patterns of inheritance
21
Pedigrees
A pedigree is a record of the features of family members. It is often shown as a diagram.
You might call this diagram a family tree. It can help you to examine and explain the
inheritance of the feature.
A pedigree is quite easy to draw if you follow the instructions below.
Constructing a pedigree
1
2
Start with the parents.
Use the male and female
symbols shown.
Draw all the offspring of
these parents using the same
symbols for male and female.
male
female
parents
son
3
For each offspring that has had
children, draw in their partner,
husband or wife.
parents
son
22
daughter
daughter daughter’s
husband
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4
Repeat step 2 for the next generation.
F (parents)
father
mother
F1 (first generation)
son
daughter
daughter’s husband
F2 (second generation)
daughter’s children
Notice that the son has no children but the daughter has two sons and one daughter.
An example of a family pedigree
From the description of the Vassallo family below, draw the pedigree.
My mother and father had me first (a boy) and then they had my two sisters.
The first born sister has one daughter. The next sister has a son and a
daughter. I have two daughters.
Check your answer.
Other pedigrees
Part 2: Patterns
of inheritance
Some
people
have more complicated families, as in the example shown below.23
my father
my adopted
older sister
my mother’s
second husband
my mother
me
my sister
my brother
half
sister
half
brother
second
husband’s
first wife
step sister
Look at the differences between a sister, adopted sister, half sister and stepsister.
Now try drawing your own family pedigree in the space below.
Using a pedigree to investigate atrait
To investigate the inheritance of a feature, you draw up a pedigree then colour in each
member who displays that trait. Here are the pedigree symbols to use.
Standard set of pedigree symbols
male without the trait being investigated
male with the trait being investigated
female without the trait being investigated
female with the trait being investigated
24
Notice how the individuals with the trait are coloured in.
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Hair form in the Vassallo family
Suppose you were to investigate the inheritance of the trait for curly hair in
the Vassallo family. (You drew their family tree in an earlier activity.)
The person who wrote the original statement says that both he and his father
have curly hair and so does his second daughter and his first sister.
His second sister has straight hair but her husband and both their children
have curly hair.
Shade in the symbols for people with the curly hair trait on the pedigree
below.
my parents
my mother
my father
me and my sisters
me
my sisters
my niece
our children
my children
my nephew
my niece
Check your answer.
How are pedigrees used?
Taronga Zoo and zoos all around the world use pedigrees to record the mating of animals in
their breeding programs. In this way, zoos can prevent diseases that occur when closely related
organisms reproduce. (This is called inbreeding and tends to cause disease because it is more
likely that related parents will carry a recessive gene for the same disease.)
To prevent inbreeding, zoos plan outbreeding programs by including unrelated animals from
other zoos around the world.
The royal family of Europe
A famous example of inbreeding is shown in the pedigree below. Royal families traditionally
marry other royal people. These means that many closely related people have reproduced
together in the royal families of Europe.
The pedigree below shows members of the royal families of Europe since Queen Victoria
(mid 1800s). It shows all the members with a genetic disease called haemophilia.
Haemophilia sufferers are unable to produce Factor 8, which is responsible for the clotting
ability of blood. This means that even a small injury can cause death due to excessive
bleeding. The inheritance of this trait can be examined and even predicted by studying this
pedigree.
Part 2: Patterns of inheritance
25
Prince
Albert
EdwardVII Alexandra
Victoria
Alice
Alfred
Queen
Victoria
Helena Louise Arthur
Leopold
Bertrice
Frederick
William
Mary
George V
Irene
Victoria
Nicholas II
of Russia
Alice
Alexandra
Alfonso XII Victoria Leopold Maurice
of Spain
Eugenia
Prince
Andrew of
Greece
?
George VI
Margaret
Elizabeth
Alexis
died in Alfonso
Rupert
(Tsarevich) (Viscount infancy (Crown
Trematon)
Prince of
Asturias)
Alice Waldermar Henry of
of Prussia Prussia
Gonzalo
Philip
(Duke of Edinburgh)
Elizabeth II
Charles Anne Andrew Edward
Using a pedigree to make predictions
Sometimes you can determine the genotype of the parents by looking at the children’s
phenotypes as well as those of the parents. A pedigree is a useful tool for solving these kinds
of problems.
For example, a blue-eyed male (recessive trait) has four children with a brown-eyed female.
Two of the children have blue eyes. You can use a monohybrid cross to determine that the
mother was a hybrid, or carrier of blue eyes, giving them a 50 : 50 chance of having blue-eyed
children.
1
Here is an example of a family whose pedigree shows members with
blue eyes (the coloured in members of the pedigree).
parents
F
children
F1
grand
children
F2
26
2
1
3
10
5
4
11
12
6
13
8
7
14
15
16
9
18
17
a) Predict the phenotype for 10, 11 and 12. Explain your answer.
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__________________________________________________
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__________________________________________________
__________________________________________________
__________________________________________________
b) Predict the genotype for 6. Explain your answer.
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
2
Study the pedigree below.
F
2
1
F1
3
5
4
6
8
7
9
F2
10
11
12
13
14
15
16
17
18
a) Is the trait being investigated dominant or recessive? Explain.
__________________________________________________
__________________________________________________
__________________________________________________
b) What are the chances of each of the first generation having the recessive genotype?
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
c) If the parents just kept on having children, what chance would there be of the
dominant trait eventually showing through?
__________________________________________________
__________________________________________________
__________________________________________________
Part 2: Patterns of inheritance
27
d) What could be the possible genotypes for each of the members numbered below?
2
_______
4
______
8
_______
11 _______
3
_______
6
______
9
_______
15 _______
Dominant traits can get a bit trickier because you need to determine whether the character
being investigated is homozygous or heterozygous. Studying a pedigree of the family can
help you to do this.
3
Consider this example.
F
2
1
F1
3
5
4
6
8
7
9
F2
10
11
12
13
14
15
16
17
18
a) The trait being studied in this pedigree is recessive. The parents
(1 and 2) do not have the trait yet two of their children (5 and 7)
do. Explain the inheritance of the recessive trait in 5 and 7.
__________________________________________________
__________________________________________________
b) Is 9 homozygous or heterozygous? Explain.
__________________________________________________
__________________________________________________
c) Is 6 homozygous or heterozygous? Explain.
__________________________________________________
__________________________________________________
d) How could you determine if 4 is homozygous or heterozygous?
__________________________________________________
__________________________________________________
Check all your answers.
Complete Exercise 2.4.
28
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Hybridisation
By now you should have a bit of an idea of what hybrid means in genetics. You learnt that
the hybrid form of a trait results from the inheritance of two different alleles.
In horticulture and agriculture, hybrid has a much less specific meaning.
A hybrid is produced from parents with very different characteristics. Offspring produced from
two quite different varieties of a species or even two different species (as in the case of a mule)
will be hybrids of those parents. Producing hybrids is a very common practice amongst plant
breeders. For example, Triticale is a crop plant formed by the hybridisation of wheat and rye. It
has the advantages of being high yielding, drought and disease resistant. It also produces fertile
offspring.
Triticale is an example of a species produced by crossing wheat and rye. (Photo: J Haeusler)
Hybridisation is the practice of selecting parent stocks with different characteristics on purpose
in order to obtain more desirable features in the offspring. There are many advantages of using
hybridisation. As soon as farmers began to produce hybrid crops, their productivity increased.
Plant nurseries sell lots of popular varieties that are hybrids.
The disadvantages of hybridisation stem from the nature of hybrids. Propagation is usually
hampered by the fact that the offspring have such diverse hereditary characteristics that they
may be infertile or not have the desired features from their parents.
An open-ended investigation of plant hybrids
Visit your nearest plant nursery or ring them. Ask if you may speak with a
nurseryman
Part 2: Patterns
of inheritanceor someone who knows about hybrids.
29
Ask the following questions and make a basic summary of the responses
in a table. (You need to draw up a table below to record your answers.)
•
What are the names of at least five different hybrid plants on sale at the moment?
•
What feature of a hybrid makes the plant more desirable than its parent stock?
•
Are the hybrids fertile or sterile?
Now use the information you have collected to identify an advantages and/or disadvantage of
hybridisation.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
30
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Suggested answers
Observing traits
1
Plants with round seeds were crossed with plants with wrinkled seeds. This is usually
written in the form:
round seeds wrinkled seeds.
2
The offspring plants all produce round peas.
3
The hybrid character is round peas (which all have the round factor and the wrinkled
factor).
Dominant and recessive traits
1
The recessive factor is not destroyed, only hidden in the first generation. It shows up in
the second generation because the recessive factor is passed on, unchanged, to some
offspring.
2
The ratios are shown in the most simplified form.
Expt
Trait examined
Dominant
factor
Recessive
factor
Ratio in second
generation
1
seed shape
round
wrinkled
3:1
2
seed colour
yellow
green
3:1
3
flower colour
violet-red
white
3:1
4
form of seed pods
inflated
constricted
3:1
5
colour of seed pods
green
yellow
3:1
6
position of flower
axial
terminal
3:1
7
length of stem
long
short
3:1
3
Yes, the ratios for traits in all the second generations are very similar.
4
dominant : recessive = 3 : 1
5
A large number of peas were needed to give a large sample size. This means that enough
information could be gathered to provide a reliable ratio.
Part 2: Patterns of inheritance
31
Explaining Mendel’s observations
Form
Factors
pure dominant
RR
pure recessive
rr
hybrid
Rr
1
Tt
2
long-stemmed : short-stemmed = 787 : 277 = 3 : 1
3
One quarter will be TT.
Two quarters (one half) will be Tt.
One quarter will be tt.
Monohybrid crosses
aa
Aa
Aa
aa
Aa
Aa
Half of the offspring are Aa hybrids, showing the dominant feature. Half of the offspring are
aa, showing the recessive feature.
Homozygous and heterozygous
1
a) bb
b) BB
c) Bb
2
Both parents have Bb genes. Therefore, their offspring are likely to be a quarter BB, a
half Bb and a quarter bb. That is, there is one chance in four, or a 25% chance, that a
baby will have blue eyes.
Genes and alleles
1
F (C, D and E may have identical alleles but you cannot be certain)
2
A and B (C, D and E may have different alleles but you cannot be certain)
3
F (C, D and E may be homozygous but you cannot be certain)
4
A and B (C, D and E may be heterozygous but you cannot be certain)
Genotype and phenotype
1
32
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2
Phenotype
Genotype(s)
round seeds
RR, Rr
wrinkled seeds
rr
The R gene is dominant so RR and Rr both produce round seeds; the recessive gene is
hidden in the hybrid form. The homologous recessive gene pair, rr, only produces
wrinkled seeds.
Mr Punnett and his squares
1
B
b
B
BB
Bb
B
BB
Bb
+
Half of the offspring have a genotype of BB and half are Bb. This means that all the
offspring will have the phenotype of brown eyes (because B is the dominant gene).
2
B
B
b
Bb
Bb
b
Bb
Bb
+
The person will be brown-eyed with a genotype of Bb.
3
B
b
b
Bb
bb
b
Bb
Bb
+
Part 2: Patterns of inheritance
33
The offspring will have either blue eyes with a genotype of bb or have brown eyes with a
genotype of Bb.
4
Two recessive genes (bb) produce the blue-eyed phenotype. However, the dominant
gene hides the recessive gene, so the hybrid form (Bb) causes the brown-eyed phenotype.
Two dominant genes (BB) also produce the brown-eyed phenotype.
An example of a family pedigree
my parents
my mother
my father
me and my sisters
me
my sisters
my niece
our children
my children
my nephew
my niece
Using a pedigree to investigate a trait
my parents
my mother
my father
me and my sisters
me
my sisters
my niece
our children
my children
my nephew
my niece
Using a pedigree to make predictions
1
a) 10, 11 and 12 will have blue eyes. 3 and 4 have blue eyes so they must both have
genotypes of bb since the blue-eyed gene is recessive. bb bb will produce all bb
offspring.
b) Male 6 is brown-eyed so his genotype is either BB or Bb. He has a blue-eyed son so
6 must be heterozygous (Bb).
2
34
a) The trait shown in the pedigree is dominant. If it were recessive, all the people in the
pedigree would have the characteristic since 1 and 2 have the trait. However, 1
and/or 2 have hidden genes that are expressed in the pedigree so they must be
Blueprint of life
hybrids showing the dominant phenotype.
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b) All of the children in F1 (4, 5, 7 and 8) have the recessive genotype. To produce a
recessive genotype, both parents (1 and 2) must be hybrids. Thus, the chances of each
person in the first generation having the recessive genotype are one chance in four, or
25%.
c) A big chance! The expected ratio of dominant : recessive = 3 : 1. In a large enough
family, this ratio should be observed.
d) 2
Aa
4
aa
8
aa
11 Aa
3
Aa
6
Aa or
AA
9
Aa
15 Aa
Part 2: Patterns of inheritance
35
3
a) Both parents must be heterozygous for the trait. A quarter of their children would be
homozygous dominant and half would be heterozygous; all these children would
show the dominant phenotype. A quarter of their children would be homozygous
recessive, like 5 and 7, and exhibit the recessive phenotype.
b) Person 9 must be heterozygous. He carries the recessive gene since he has passed it
to his elder son and to his daughter. He exhibits the dominant phenotype so he must
also have the dominant gene.
c) Person 6 displays the dominant phenotype so he could be homozygous dominant or
heterozygous. There are not enough children in the family to be sure that one child
will not display the recessive trait.
d) Look at person 4’s children. If she has any children with the recessive trait (which
she does) then she must carry the recessive gene hidden by a dominant gene. She is
heterozygous.
If she had many children and none of them had the recessive trait, then perhaps she
could be homozygous. However, she could still be heterozygous if person 3 is
homozygous dominant. You would need to investigate person 3 to find out whether
they are homozygous or heterozygous before you could make a decision.
36
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Exercises – Part 2
Exercises 2.1 to 2.4
Name: _________________________________
Exercise 2.1: Recognition of Mendel’s work
a)
Outline the experiments carried out by Gregor Mendel. Include information about each
of the following in your answer.
•
What kind of organism, and how many of them, did he study?
•
What kinds of features did he study?
•
How many features did he study at one time?
•
How did he perform his investigations? (What was he studying?)
•
What did he do that was similar to other researchers?
•
What did he do that was different?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
b) Mendel’s research was not recognised for about 35 years after it was published. What are
three reasons why this happened?
• ____________________________________________________
____________________________________________________
____________________________________________________
Part 2: Patterns of inheritance
37
• ____________________________________________________
____________________________________________________
____________________________________________________
• ____________________________________________________
____________________________________________________
____________________________________________________
c)
What was the main factor that led to the rediscovery of Mendel’s work?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
38
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Exercise 2.2: Genes and alleles
Here is a diagram representing peas produced by pea plants with different genetic
information.
round (RR)
a)
round (Rr)
wrinkled (rr)
What trait is shown?
_____________________________________________________
_____________________________________________________
b) How many genes are responsible for this trait? What are they?
_____________________________________________________
_____________________________________________________
_____________________________________________________
c)
Are these genes alleles? Explain.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
d) Circle the group(s) of peas above that are homozygous for this trait. Explain why you
chose these peas.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 2: Patterns of inheritance
39
Exercise 2.3: Mr Punnett and his squares
The trait of plant height is controlled by two genes – T for tall and
t for short.
a)
Determine the possible genotypes and phenotypes for the following crosses. Show all
your working using Punnett squares.
X
homozygous tall
short
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
X
heterozygous tall
40
heterozygous tall
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_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
b) Why are the pea plants either tall or short in each of these crosses?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 2: Patterns of inheritance
41
Exercise 2.4: Using a pedigree to make predictions
One form of deafness in humans is due to a dominant gene, D. Normal hearing depends on a
recessive gene, d. The D gene is very uncommon in Australia’s population.
Joe, who is deaf and has the D gene, marries Sue, who has normal hearing, and they have four
children. The eldest child, John, and the eldest daughter, Kim, are deaf. The other two
daughters, Kate and Julie, have normal hearing.
John is married to a deaf woman, Sally, and they have three sons. Julie is married to a
hearing man, Sam, and they have a daughter then a son.
a)
Draw a pedigree for this family, showing the trait of deafness.
b) What is the chance of Julie and Sam having a deaf child? Explain.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
c)
Would you expect John and Sally’s children to be deaf or hearing? Explain. (There are
three possibilities that you need to discuss in your answer.)
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
42
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_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
d) How do you think the pedigree you have drawn could help this family?
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 2: Patterns of inheritance
43