4.3 Theoretical Genetics
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Topic 4: Genetics
4.3 Theoretical Genetics
4.3.1 Define genotype, phenotype, dominant allele, recessive allele, codominant
alleles, locus, homologous, heterozygous, carrier and test cross (pg. 137-139. Pg.
59).
4.3.2 Determine the genotypes and phenotypes of the offspring of a
monohybrid cross using a Punnett grid (pg. 137, pg. 59).
4.3.3 State that some genes have more than two alleles (multiple alleles) (Pg.
141, pg. 60)
4.3.4 Describe ABO blood groups as an example of codominance and multiple
alleles (Pg. 140, pg. 60).
4.3.5 Explain how the sex chromosomes control gender by referring to the
inheritance of X and Y chromosomes in humans (pg. 145).
4.3.6 State that some genes are present on the X-chromosome and absent
from the shorter Y chromosome in humans (pg. 145).
4.3.7 Define sex linkage (pg. 144).
4.3.8 Describe the inheritance of colour blindness and haemophilia as examples
of sex linkage (pg. 144).
4.3.9 State that a human female can be homozygous or heterozygous with
respect to sex-linked genes (pg. 144).
4.3.10 Explain that female carriers are heterozygous for X-linked recessive
alleles (pg. 144).
4.3.11 Predict the genotypic and phenotypic ratios of offspring of monohybrid
crosses involving any of the above patterns of inheritance (pg. 137-140).
4.3.12 Deduce the geneotypes and phenotypes of individuals in pedigree charts
(pg. 138).
4.3.1 Definitions
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4.3.1 Define genotype, phenotype, dominant allele, recessive allele, codominant
alleles, locus, homologous, heterozygous, carrier and test cross
Orange book  pg. 137-139
Green book  pg. 59
To do:
Give a definition for each of the terms in the objective.
Genotype
Phenotype
Dominant Allele
Recessive Allele
Codominant Allele
Locus
Homologous
Heterozygous
Carrier
Test Cross
4.3.2 Monohybrid Crosses
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4.3.2 Determine the genotypes and phenotypes of the offspring of a
monohybrid cross using a Punnett grid.
Orange book  pg. 137
Green book  pg. 59
To do:
Look at the example shown showing the cross between a red flower and a white
flower.
Follow this through on the board and take it down in your green books.
Visit the website:
http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html
Go to “Genetic Crosses”  “Monohybrid and Dihybrid” (we will focus on
monohybrid as dihybrid is higher level).
The site provides multiple choice genetic questions.
In your green books – wok out the answers using a Punnett square as shown in
the example below.
If you get stuck  try the link for a tutorial at the bottom of the page to help
explain the problem.
The Monohybrid Cross - Example
A simple breeding experiment involving just a single characteristic (controlled
by one gene) is called a monohybrid cross.
At fertilisation any male gamete can fertilise any female gamete at random.
The possible results of a fertilisation can be worked out using a Punnett Square
as shown in the diagram below.
4.3.3 Multiple Alleles
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4.3.3 State that some genes have more than two alleles (multiple alleles).
Orange book  pg. 141
Green book  pg. 60
To do:
Read the paragraph below and define “codominance”.
Follow the example through on the board (the same example as below:
codominance in flower colour) and take notes on the cross.
Follow the second example (sickle cell anaemia) through on the board and take
notes on the cross.
Answer the questions in your exercise book.
Codominance
In most situations (and all of Mendel’s experiments) one allele is completely
dominant over the other, so there are just two phenotypes. But in some cases
there are three phenotypes, because neither allele is dominant over the other,
so the heterozygous genotype has its own phenotype. This situation is called
codominance. Since there is no dominance we can no longer use capital and small
letters to indicate the alleles, so a more formal system is used. The gene is
represented by a letter, and the different alleles by superscripts to the gene
letter.
Codominance: Flower colour
A good example of codominance is flower colour in snapdragon (Antirrhinum)
plants. The flower colour gene C has two alleles: CR (red) and CW (white). The
three genotypes and their phenotypes are:
The monohybrid cross looks like this:
Codominance: Sickle Cell Anaemia
Another example of codominance is sickle cell haemoglobin in humans. The gene
for haemoglobin Hb has two codominant alleles: HbA (the normal gene) and HbS
(the mutated gene). There are three phenotypes:
HbAHbA Normal  All haemoglobin is normal, with normal red blood cells.
HbAHbS Sickle cell trait  50% of the haemoglobin in every red blood cell is
normal, and 50% is abnormal. The red blood cells are slightly distorted, but can
carry oxygen, so this condition is viable. However these red blood cells cannot
support the malaria parasite, so this phenotype confers immunity to malaria.
HbSHbS Sickle cell anaemia  All haemoglobin is abnormal, and molecules stick
together to form chains, distorting the red blood cells into sickle shapes.
These sickle red blood cells are destroyed by the spleen, so this phenotype is
fatal.
Other examples of codominance include coat colour in cattle (red/white/roan),
and coat colour in cats (black/orange/tortoiseshell).
Questions – to be answered in exercise books
1. Some cats have white patches on their coats. This effect is produced by the
action of the spotting gene, S. This gene has two codominant alleles, S1 and S2.
The coats can have large patches of white, small white patches or no white
patches at all.
(a) Explain the meaning of the term codominance. (2)
(b) A cat with no white patches, homozygous for S1, was mated with a cat that
had small white patches. Some of the offspring produced had coats with small
white patches and the rest had no white patches.
Draw out the genetic diagram to show this cross and include the expected ratio
of phenotypes on your diagram. (4)
2. Chickens homozygous for black feathers were crossed with chickens
homozygous for white feathers. These colours are determined by alleles of a
single gene (F). All the first generation offspring had blue feathers. When the
blue-feathered first generation chickens were crossed with each other, there
were black-feathered, white-feathered and blue-feathered chickens in the
second generation offspring.
(a) Complete a genetic diagram to explain how the first and second generation
phenotypes were produced (4)
(b) The number of black-feathered, white-feathered and blue-feathered
chickens in the second offspring was counted. The observed ratio of black :
white : blue was similar to the ratio expected from theory but not the same.
Explain why observed ratios are often not the same as the expected ratios. (2)
4.3.4 Blood Groups
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4.3.4 Describe ABO blood groups as an example of codominance and multiple
alleles.
Orange book  pg. 140
Green book  pg. 60
To do:
Read the paragraph below and define “ABO Blood Groups”.
Follow the example through on the board (Crossing Group A - heterozygous with
Group B - heterozygous) and take notes on the cross.
ABO Blood Groups
An individual has two copies of each gene, so can only have two alleles of any
gene, but there can be more than two alleles of a gene in a population. An
example of this is blood group in humans.
The red blood cell antigen is coded for by the gene I (for isohaemaglutinogen –
don’t need to know), which has three alleles IA, IB and IO. (They are written this
way to show that they are alleles of the same gene).
IA and IB are codominant, while IO is recessive. The possible genotypes and
phenotypes are:
Example: Crossing Group A (heterozygous) with Group B (heterozygous)
The cross below shows how all four blood groups can arise from a cross between
a group A and a group B parent.
Questions
1. Distinguish between the terms gene and allele. (2)
2. The diagram below shows a family tree in which the blood group phenotypes
are shown for some individuals.
(a) Using the symbols IA, IB and I° to represent the alleles, indicate the
genotypes of the following people: 1, 2, 4, 5 and 6. (5)
(b) State the possible blood groups of person 3. Explain your answer. (3)
3. The human ABO blood groups are an example of multiple alleles, where three
or more alleles occur at a gene locus. The allele IO is recessive and the alleles IA
and IB are co-dominant.
(a) What does the term co-dominant refer to? (1)
(b) A couple have four children. Two children have blood group O, the other two
children have blood group A. Give the most likely genotypes and phenotypes for
the parents. (2)
(c) A mother has a blood group AB, the father is group A. What is the
probability of their first child having blood group AB if the father is: (2)
Homozygous?
Heterozygous?
4.3.5 Sex Determination
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4.3.5 Explain how the sex chromosomes control gender by referring to the
inheritance of X and Y chromosomes in humans.
Orange book  pg. 145
Green book  pg. 60-61
To do:
Using a genetic diagram and a short paragraph, explain sex determination in
humans.
Sex Determination
Sex is determined by the sex chromosomes (X and Y). In humans the sex
chromosomes are homologous in females (XX) and non-homologous in males (XY),
though in other species it is the other way round. The inheritance of the X and
Y chromosomes can be demonstrated using a monohybrid cross:
This shows that there will always be a 1:1 ratio of males to females. Note that
female gametes (eggs) always contain a single X chromosome, while the male
gametes (sperm) can contain a single X or a single Y chromosome. Sex is
therefore determined solely by the sperm. There are techniques for separating
X and Y sperm, and this is used for planned sex determination in farm animals
using IVF.
4.3.6 Sex Chromosomes
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4.3.6 State that some genes are present on the X-chromosome and absent from
the shorter Y chromosome in humans.
Orange book  pg. 145
Green book  pg. 61
4.3.7 Sex Linkage
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4.3.7 Define sex linkage.
Orange book  pg. 144
Green book  pg. 61
To do:
Read the paragraph below “Sex Linkage”
In one sentence, define Sex Linkage
Follow the example of eye colour in fruit flies through on the board and copy
the notes into your exercise book.
Answer the questions at the end in your exercise books.
Visit the website:
http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html
Go to “Genetic Crosses”  “Sex-linked Inheritance 1 and 2”.
The site provides multiple choice genetic questions.
In your green books – work out the answers using Punnett squares.
If you get stuck  try the link for a tutorial at the bottom of the page to help
explain the problem.
Sex Linkage
The X and Y chromosomes don’t just determine sex, but also contain many other
genes that have nothing to do with sex determination. The Y chromosome is
very small and seems to contain very few genes, but the X chromosome is large
and contains thousands of genes for important products such as rhodopsin (for
sight), blood clotting proteins and muscle proteins. Females have two copies of
each gene on the X chromosome (i.e. they’re diploid), but males only have one
copy of each gene on the X chromosome (i.e. they’re haploid). This means that
the inheritance of these genes is different for males and females, so they are
called sex-linked characteristics.
Example: Eye Colour in Fruit Flies
The first example of sex-linked genes discovered was eye colour in Drosophila
fruit flies. Red eyes (R) are dominant to white eyes (r) and when a red-eyed
female is crossed with a white-eyed male, the offspring all have red eyes, as
expected for a dominant characteristic (left cross below).
However, when the opposite cross was done (a white-eye female with a red-eyed
male) all the male offspring had white eyes (right cross below). This surprising
result was not expected for a simple dominant characteristic, but it could be
explained if the gene for eye colour was located on the X chromosome.
Note that in these crosses the alleles are written in the form XR (red eyes) and
Xr (white eyes) to show that they are on the X chromosome.
Males always inherit their X chromosome from their mothers, and always pass
on their X chromosome to their daughters.
Sex-Linkage Questions – answer in exercise books
1. One of the genes that controls coat colour in cats has its locus on the X
chromosome. The alleles are XO, giving orange coat colour, and XB, giving black
coat colour. The genotype XOXB produces a coat with a mixture of orange and
black, known as tortoiseshell.
A female cat with orange coat colour was mated with a black male. The resulting
offspring were tortoiseshell females and orange males.
(a) With reference to the alleles XO and XB explain why all the male kittens
from this cross had orange coats. (3)
(b) The black male cat was later mated with a tortoiseshell female. By means of
a genetic diagram, show the possible genotypes and phenotypes of the offspring
of this cross. (4)
2. Fabry’s disease is a sex-linked recessive genetic disorder that causes mental
retardation.
A study was carried out into the inheritance of this disorder in a family, and the
results are shown in the pedigree below.
(a) Using the symbol A for the dominant allele and a for the recessive allele,
state the genotype of person 2. (1)
(b) Using the evidence from the pedigree, explain why Fabry’s disease is
described as a sex-linked recessive genetic disorder. (3)
(c) Explain why person 3 is unaffected but why one of his children (person 5)
has Fabry’s disease. (3)
(d) What are the chances of the children of persons 6 and 7 having Fabry’s
disease? Give reasons for your answer. (4)
3. In the fruit fly, Drosophila melanogaster, the allele r for white eyes is sex–
linked and is recessive to the allele R for the normal red eye colour. Sex–linked
genes are carried on the X chromosome. In fruit flies, the sex chromosomes are
XX in females and XY in males.
When a white–eyed female fly is crossed with a red–eyed male, all of the male
offspring have white eyes.
(a) Draw diagrams to represent the chromosomes, and alleles present on them,
of the parents in this cross. (3)
(b) State the genotype and phenotype of the female offspring. (2)
A gene which affects the size of the wings in fruit flies is not sex–linked
(autosomal).
The allele for short (dumpy) wings, d, is recessive to the allele, D, for normal
wings.
A female fly was heterozygous for the genes for eye colour and for wing size.
This female was crossed with a white–eyed, short–winged male.
(c) Use a genetic diagram to show the expected results of this cross. (4)
(d) Using the table below, list the genotypes and full phenotypes of the male
offspring produced. (2)
4. Becker muscular dystrophy is an inherited condition caused by an allele of a
gene. Sufferers experience some loss of muscle strength. The diagram shows
how members of one family were affected by the condition.
(a) Explain one piece of evidence from the diagram which shows that the allele
for Becker muscular dystrophy is recessive. (2)
(b) The allele for Becker muscular dystrophy is sex-linked. Explain how
individual 9 inherited the condition from his grandfather. (2)
4.3.8 Sex-linked Conditions
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Describe the inheritance of colour blindness and haemophilia as examples of sex
linkage.
Orange book  pg. 144
Green book  pg. 61-62
Colour Blindness
A well-known example of a sex linked characteristic is colour blindness in
humans. 8% of males are colour blind, but only 0.7% of females. The genes for
green-sensitive and red-sensitive rhodopsin are on the X chromosome, and
mutations in either of these lead to colour blindness. The diagram below shows
two crosses involving colour blindness, using the symbols XR for the dominant
allele (normal rhodopsin, normal vision) and Xr for the recessive allele (nonfunctional rhodopsin, colour blind vision).
Other examples of sex linkage include haemophilia, premature balding and
muscular dystrophy.
4.3.9 Females & Sex-linkage
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State that a human female can be homozygous or heterozygous with respect to
sex-linked genes.
Orange book  pg. 144
Green book  pg. 62
To do:
 Read the relevant sections in the book.
 Explain the objective in your own words.
4.3.10 Heterozygous females
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4.3.10 Explain that female carriers are heterozygous for X-linked recessive
alleles.
Orange green  pg. 144
Green book  pg. 62
 Read the relevant sections in your book.
 Explain the objective in your own words.
4.3.11 Monohybrid Crosses
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4.3.11 Predict the genotypic and phenotypic ratios of offspring of monohybrid
crosses involving any of the above patterns of inheritance (pg. 137-140).
Orange book  pg. 137-140
Green book  pg. 62-63

If you have made it this far you have completed all the crossed you need to do.
4.3.12 Pedigree Charts
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4.3.12 Deduce the geneotypes and phenotypes of individuals in pedigree charts.
Orange book  pg. 138
Green book  pg. 63
Family pedigrees show how genetic traits can be followed through a family tree.
When considering the possible genotypes of people within a family tree it is
best to start with those you definitely know – homozygous recessive. From
there it is best to work from children backwards towards their parents.
E.g. Brown eyes (B) is dominant to blue eyes (b). If a child was born with blue
eyes and yet both its parents had brown eyes, you could conclude that the
parents must be heterozygous (Bb) as they each must be able to pass on the
allele for blue eyes.
So the steps you should follow are:
1. Identify those genotypes you are certain about = homozygous recessive.
2. For dominant characteristics you can add in one allele e.g. B_.
3. Then look at children to help determine parental genotype e.g. what alleles
must parents have carried to provide the child with the genotype they have.
Questions  answer in green exercise book.
1. The allele for brown hair (B) is dominant to the allele for blond hair (b).
The diagram shows the inheritance of hair colour in a family.
(a) Using the symbols B and b, state the genotype of: (2)
Jim
Jane
(b) What is Amanda’s phenotype? (1)
(c) How many people in the family are homozygous? (1)
2. The black pigment in human skin and eyes is called melanin.
A single gene controls the production of melanin.
A person who is homozygous for the recessive allele of the gene has no melanin
and is said to be albino.
The diagram shows the inheritance of albinism in a family.
(a) Use a genetic diagram to explain the inheritance of the albino allele by
children of parents P and Q. (3)
(b) R and S decide to have a child. What is the chance that this child will be an
albino? Use a genetic diagram to explain your answer. (3)
3. The diagrams show a hand and foot from a person with polydactyly.
Polydactyly is a genetic condition determined by a dominant allele.
(a) What is the phenotype of someone who is homozygous dominant? (1)
(b) What is the phenotype of someone who is homozygous recessive? (1)
The diagram shows the inheritance of polydactyly in a family. The allele for
polydactyly is shown as D and the allele for being unaffected is shown as d.
(c) What is the sex and phenotype of Sam? (2)
(d) What are the possible genotypes for Emma? (2)
(e) Wayne has polydactyly. What is his genotype? (1)
(f) How was Sam’s sex determined at fertilisation? (2)
(g) These parents are expecting another child. Use a genetic diagram to work
out the probability that this child will have polydactyly. (3)
4. The genetic diagram shows two parents and three children.
Only the son has cystic fibrosis, which is caused by a recessive allele. What
conclusion may be made about the parents’ genes? (1)
5. Huntington’s disease is a rare inherited disorder of the nervous system. It is
caused by a dominant allele H. The recessive allele of this gene is represented
by h. The diagram shows the inheritance of Huntington’s disease in a family.
(a) Use a genetic diagram to show the inheritance of the Huntington’s disease
allele by the children of parents P and Q. (3)
(b) Explain why none of the children of R and S inherited Huntington’s disease.
(2)
6. Cystic fibrosis is an inherited disease where certain cells produce abnormal
mucus. The allele for the disease is recessive. The diagram shows how cystic
fibrosis was inherited in one family.
(a) Complete the diagram by correctly shading the symbols for J and K. (1)
(b) Persons A and B are said to be ‘symptomless carriers’. What does this
mean? (2)
(c) How many of the children of A and B were homozygous dominant? (1)
(d) What is the phenotype of D? (1)
(e) What is the probability of F and G having a child with cystic fibrosis? (1)
(f) What is the probability of F and G having a female child with cystic fibrosis?
(1)
7. PKU (phenylketonuria) is an inherited disease. The allele (n) for the disease is
recessive to the normal allele (N). The diagram shows how PKU was inherited in
a family.
Individual
B
J
Genotype
(a) Give the genotype of each individual in the table below. (2)
(b) How many of the children of A and B are homozygous? (1)
(c) If G and H have a child, what is the probability that it will have PKU? (1)
(d) C and D have four children, all of whom are female. What is the probability
that their next child will be female? (1)
8. Thalassaemia is a genetic disease caused by a recessive allele. People may be
carriers of thalassaemia, without showing any of the symptoms. Colin and Clare
plan to marry. As there is a history of thalassaemia in both families, Colin and
Clare undergo a pedigree analysis. They want to know if they are carriers of
the thalassaemia allele. The result of the pedigree analysis are shown in the
following family trees.
(a) Explain how the analysis shows that Colin in a carrier for thalassaemia. (2)
(b) What is the probability that Clare is a carrier? (1)