4.3 Theoretical Genetics 3/17/11 5:51 PM

Topic 4: Genetics4.3 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 3/17/11 5:51 PM

4.3.1 Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homologous, heterozygous, carrier and test cross

Orange book pg. 137-139Green 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 3/17/11 5:51 PM

4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

Orange book pg. 137Green 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.htmlGo 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 - ExampleA 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 3/17/11 5:51 PM

4.3.3 State that some genes have more than two alleles (multiple alleles).

Orange book pg. 141Green 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.

CodominanceIn 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 colourA 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 AnaemiaAnother 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 books1. 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 3/17/11 5:51 PM

4.3.4 Describe ABO blood groups as an example of codominance and multiple alleles.

Orange book pg. 140Green 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 GroupsAn 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.

Questions1. 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 3/17/11 5:51 PM

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. 145Green book pg. 60-61

To do:Using a genetic diagram and a short paragraph, explain sex determination in humans.

Sex DeterminationSex 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 3/17/11 5:51 PM

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. 145Green book pg. 61

4.3.7 Sex Linkage 3/17/11 5:51 PM

4.3.7 Define sex linkage.

Orange book pg. 144Green 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.htmlGo 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 LinkageThe 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 FliesThe 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 books1. 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 3/17/11 5:51 PM

Describe the inheritance of colour blindness and haemophilia as examples of sex linkage.

Orange book pg. 144Green book pg. 61-62

Colour BlindnessA 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 (non-functional rhodopsin, colour blind vision).

Other examples of sex linkage include haemophilia, premature balding and muscular dystrophy.

4.3.9 Females & Sex-linkage 3/17/11 5:51 PM

State that a human female can be homozygous or heterozygous with respect to sex-linked genes.

Orange book pg. 144Green book pg. 62

To do: Read the relevant sections in the book. Explain the objective in your own words.

4.3.10 Heterozygous females 3/17/11 5:51 PM

4.3.10 Explain that female carriers are heterozygous for X-linked recessive alleles.

Orange green pg. 144Green book pg. 62

Read the relevant sections in your book. Explain the objective in your own words.

4.3.11 Monohybrid Crosses 3/17/11 5:51 PM

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. 137-140Green 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 3/17/11 5:51 PM

4.3.12 Deduce the geneotypes and phenotypes of individuals in pedigree charts.

Orange book pg. 138Green 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)JimJane

(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.

(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)

Individual GenotypeBJ

(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)