AQA Bio A level Unit 7

Question 1

In a population, 36% of cats had the F allele and had white fur.

Use the Hardy–Weinberg equation to calculate the frequency of the f allele in this population.

Answer 1

0.8 = 2 marks

Question 2

There are four ABO phenotypes, A, B, AB and O. Name the statistical test you should use to determine if the observed frequencies of the four phenotypes differed significantly from the frequencies expected according to the Hardy–Weinberg equation.

State how many degrees of freedom should apply.

Statistical test _____________________________________________________

Number of degrees of freedom ____________________________________

Answer 2

1. Chi-squared;
2. 3;

Question 3

There are four ABO phenotypes, A, B, AB and O.

The scientists concluded that the observed frequencies of the four phenotypes differed significantly from the expected frequencies. Use your knowledge of the Hardy–Weinberg principle to suggest two reasons why.

Answer 3

1. Selection (against/for a blood group/phenotype/allele);
2. (High rate of) mutation;
3. Immigration/emigration;

Accept ‘migration’ or population is not isolated.

4. No random mating.

Ignore no ‘random fertilisation’.

Reject converse statements as context would be incorrect

Ignore births/deaths.

Question 4

In fruit flies, males have the sex chromosomes XY and the females have XX. In fruit flies, a gene for eye colour is carried on the X chromosome. The allele for red eyes, R, is dominant to the allele for white eyes, r.

Male fruit flies are more likely than female fruit flies to have white eyes. Explain why.

Answer 4

1. Males have one allele; Accept males only need one allele.
2. Females need two recessive alleles

OR Females must be homozygous recessive

OR Females could have dominant and recessive alleles

OR Females could be heterozygous/carriers;

Question 5

Mutation is one cause of genetic variation in organisms.

Give two other causes of genetic variation.

Answer 5

1. Crossing over;
2. Independent segregation/assortment (of homologous chromosomes);

Accept independent assortment of alleles.

Accept meiosis as an alternative for 1 or 2 if neither of these marks is awarded.

3. Random fusion of gametes

OR

Random fertilisation;

Question 6

In a species of flowering plant, the T allele for tallness is dominant to the t allele for dwarfness. In the same species, two alleles CR (red) and CW (white) code for the colour of flowers. When homozygous red-flowered plants were crossed with homozygous white-flowered plants, all the offspring had pink flowers.

Name the relationship between the two alleles that code for flower colour.

Answer 6

Codominance;

Question 7

In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.

Suggest three reasons why.

Answer 7

1. Small sample size;
2. Fusion/fertilisation of gametes is random;
3. Linked Genes; Accept crossing over / sex linkage
4. Epistasis;
5. Lethal genotypes;

Question 8

What is meant by the term phenotype?

Answer 8

1. (Expression / appearance / characteristic due to) genetic constitution / genotype / allele(s);
2. (Expression / appearance / characteristic due to) environment;

Question 9

Suggest two reasons why observed ratios are often not the same as expected ratios.

Answer 9

1.Fertilisation is random

OR

Fusion of gametes is random;

2. Small / not large population / sample;
3. Selection advantage / disadvantage / lethal alleles;

Question 10

This fruit fly has another characteristic controlled by a pair of codominant alleles, WN and WV.

What is meant by codominant alleles?

Answer 10

Both alleles expressed in the phenotype (if both are present);

Question 11

In fruit flies, the genes for body colour and wing length are linked. Explain what this means.

Answer 11

Genes on same chromosome.

Question 12

In cats, males are XY and females are XX. A gene on the X chromosome controls fur colour in cats. The allele G codes for ginger fur and the allele B codes for black fur. These alleles are codominant. Heterozygous females have ginger and black patches of fur and their phenotype is described as tortoiseshell. Male cats with a tortoiseshell phenotype do not usually occur. Explain why.

Answer 12

Only possess one allele / Y chromosome does not carry allele / gene / can’t be heterozygous.

Question 13

In cats, males are XY and females are XX. A gene on the X chromosome controls fur colour in cats. The allele G codes for ginger fur and the allele B codes for black fur. These alleles are codominant. Heterozygous females have ginger and black patches of fur and their phenotype is described as tortoiseshell.

A tortoiseshell female was crossed with a black male. Use a genetic diagram to show all the possible genotypes and the ratio of phenotypes expected in the offspring of this cross.

Use XG to indicate the allele G on an X chromosome.
Use XB to indicate the allele B on an X chromosome.

Genotypes of offspring ________________________________________________

Phenotypes of offspring _______________________________________________

Ratio of phenotypes __________________________________________________

Answer 13

1. XGXB, XBXB, XGY, XBY;

Accept: equivalent genotypes where the Y chromosome is shown as a dash e.g. XG-, or is omitted e.g. XG

2. Tortoiseshell female, black female, ginger male, black male;
3. (Ratio) 1:1:1:1

Question 14

In birds, males are XX and females are XY. Use this information to explain why recessive, sex-linked characteristics are more common in female birds than in male birds.

Answer 14

Recessive) allele is always expressed in females / females have one
(recessive) allele / males need two recessive alleles / males need to be homozygous recessive / males could have dominant and recessive alleles / be heterozygous / carriers;