AQA A Level Biology

Topic Questions

7.1 Inheritance (A Level only)

1a3 marks

Identify labels A-C on the chromosome shown in Figure 1.

Figure 1

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1b1 mark

Define the term genotype.

1c1 mark

Define the term sex-linked in genetics.

1d2 marks

Haemophilia is a sex-linked disease which impacts the body’s ability to form blood clots. The allele which causes haemophilia is carried on the X-chromosome. Explain why a male can only inherit haemophilia from his mother.

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2a1 mark

Gregor Mendel carried out monohybrid crosses on pea plants. Figure 1 shows an overview of the method Gregor Mendel used. Note that the term ‘pure breeding’ means that an individual is homozygous for a trait.

Figure 1

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Suggest how Gregor Mendel could have ensured that the pea plants in stage 1 of Figure 1 were pure-breeding.

2b3 marks

Use a genetic diagram to show the genotype and phenotype of the offspring in the F1 generation described in Figure 1.

2c2 marks

Gregor Mendel then crossbred two individuals from the F1 generation in Figure 1 and used the ratios to show that the allele for green pods was dominant over the allele for yellow pods. The results of the F2 cross are shown in Table 1.

Table 1

Pod colour

Number in F2 generation

Green

6020

Yellow

2029

Explain how the results in Table 1 support the theory that the allele for green seed pods is dominant and the allele for yellow seed pods is recessive.

2d1 mark

Gregor Mendel carried out thousands of crosses to confirm the ratios which backed up his theories about dominant and recessive alleles. 

Suggest why the ratios that Mendel expected were not always the same as the actual ratios that were observed.

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3a2 marks

In cats, a gene found on the X chromosome determines fur colour. One allele G codes for ginger fur and the other allele B codes for black fur. Heterozygous female cats will have both ginger and black fur, a phenotype known as tortoiseshell.

Identify and explain the type of allele interaction in this example.

3b1 mark

Using the information given in part a), state why it is not usually possible to see male tortoiseshell cats.

3c3 marks

A female tortoiseshell cat was crossed with a black male cat. 

Use a genetic diagram to calculate what percentage of their offspring are likely to be black male cats.

3d1 mark

Presence of another gene at a locus on a different chromosome results in cats with completely black fur. Identify the term used to describe this type of interaction between alleles.

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4a2 marks

In guinea pigs, the allele for black hair (B) is dominant to the allele for white hair (b) and the allele for long hair (L) is dominant to the allele for short hair (l). A double homozygous guinea pig with long, black hair was bred with another double homozygous guinea pig with long white hair.                          

State the genotypes of the two parent guinea pigs.

4b3 marks

Use a genetic diagram to show the ratio of different phenotypes which could result from the cross discussed in part a.

4c1 mark

Figure 1 shows the phenotypes in a family tree used to track a genetic condition called haemophilia.

Figure 1

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Give one piece of evidence from Figure 1 which shows that haemophilia is recessive. 

4d2 marks

Haemophilia is a sex-linked condition. Use this information and Figure 1 to Identify the genotype of person number 10 from Figure 1. Explain your answer.

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5a3 marks

Figure 1 shows five examples of different genetic interactions.

Figure 1

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Complete Table 1 below to match up the correct example with the right explanation.

Table 1

Explanation

Letter of example

One gene with two alleles. The alleles show      codominance

 

One gene with two alleles located on an autosome (gene not sex linked). One allele is dominant and the other is recessive.

 

A sex linked gene with a dominant and a recessive allele.

 

5b1 mark

Fur colour in mice is controlled by two genes. One gene (B/b) controls the expression of a specific fur colour, while production of hair pigment is controlled by another gene (C/c). The allele for black fur (B) is dominant to the allele for brown fur (b). Mice that possess the homozygous recessive genotype (cc) will lack hair pigment making them albino. This is an example of epistasis.

The punnett square in Figure 2 below shows a cross between two black mice with the genotype BbCc.

Figure 2

 

BC

bC

Bc

bc

BC

BBCC

BbCC

BBCc

BbCc

bC

BbCC

bbCC

BbCc

bbCc

Bc

BBCc

BbCc

BBcc

Bbcc

bc

BbCc

bbCc

Bbcc

bbcc

State the ratios of different phenotypes shown in this cross.

5c2 marks

Scientists completed an investigation into the incidence of albinism in mice but found that their results did not achieve the same ratios as expected.

Suggest two reasons why their ratios may not have matched up to the expected ratios

5d1 mark

In order to establish whether there was a significant difference between the actual results and the expected results, a statistical test is required.

Identify which statistical test the scientists should use.

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1a2 marks

Fur colour in rabbits is controlled by a gene inherited on the X chromosome. Some rabbits can be either brown or white. Female rabbits can be described as ‘roan’ if they have patches of brown and white fur. 

Explain why male rabbits cannot be roan. 

1b3 marks

A roan female rabbit is crossed with a white male rabbit. 

Use a genetic diagram to state the ratio of phenotypes expected in the offspring of this cross.

1c2 marks

The effects of the fur colour alleles can be further modified by a second gene which is not sex-linked. The allele r changes the brown colour to tan, and white colour to grey. Allele R has no effect on fur colour in rabbits. 

A grey coloured male rabbit was crossed with a roan female who is heterozygous for the second colour modifying gene. 

State the genotypes of both parents. 

1d4 marks

A grey coloured male rabbit was crossed with a roan female who is heterozygous for the second colour modifying gene. 

Use a genetic diagram and your answers to part c), to show the ratio of expected  phenotypes of the male offspring of this cross.

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2a3 marks

An investigation on fruit flies, Drosophila melanogaster, was carried out to determine the relationship between body colour and wing length. Scientists found the genes controlling these characteristics are inherited on different autosomal chromosomes. Fruit flies are either black or grey and have either long or short wings. 

Two homozygous fruit flies were crossed: a fruit fly with a black body and long wings was crossed with a grey fruit fly with short wings. All of the offspring produced had black bodies with long wings. 

Using this information and a genetic diagram, show how these offspring were produced. Use B/b and L/l to represent the alleles. 

2b3 marks

The offspring from part a) were crossed with grey bodied fruit flies with short wings. Use a genetic diagram to show the expected ratios of the phenotypes expected from this cross.

2c1 mark

The scientists determined the offspring phenotypes from the cross in part b) and the data collected is shown in Table 1

Table 1

Phenotype

Observed Numbers

Expected Numbers

Black body and long wings

83

 

Black body and short wings

85

 

Grey body and long wings

78

 

Grey body and short wings

74

 

Table 1 showing the observed number of offspring produced for each phenotype

Complete Table 1 to show the expected number of offspring for each phenotype.

2d5 marks

The scientists claimed that independent segregation had taken place. Chi2 can be used to determine the significance of the data. The critical value for this data is 7.82. Use the following formula to calculate the value of Chi squared (χ2) and comment on the scientists claim. 

    x squared equals sum fraction numerator open parentheses o b s e r v e d minus e x p e c t e d close parentheses squared over denominator e x p e c t e d end fraction

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3a2 marks

A scientist crossed a grey male cat with a grey female cat multiple times. There were 12 kittens born of which 8 were grey and 4 were white. The trait is not sex-linked. 

Explain how the information provides evidence that white fur is recessive. 

3b1 mark

Two cats were crossed, producing 11 kittens. Five were grey and six were white. Use this information to determine the genotype of the parents.

3c3 marks

There are three alleles that control fur colour in cats. The allele G gives grey fur, g gives black fur, and the allele gi gives ginger fur. Allele G is dominant to both of the other alleles, and gi is recessive to g

Scientists crossed a black male several times with a grey female. The offspring were as follows:

  • 10 grey kittens
  • 3 black kittens
  • 4 ginger kittens

Draw a genetic diagram to show how these offspring were produced.

3d3 marks

Scientists expected an equal number of black and ginger kittens. Explain why the numbers of actual black and ginger kittens are different to expected. 

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4a3 marks

Muscular dystrophy is a recessive genetic condition that causes muscles to weaken over time. There are multiple forms of muscular dystrophy that can be inherited. One of these forms is inherited on the X chromosome. The dominant allele results in normal muscle formation. 

Figure 1 shows the pattern of inheritance of these alleles in one family. 

Figure 1ObauP~j__1

Use evidence from the diagram to explain that muscular dystrophy is caused by the recessive allele and inherited on the X chromosome.  

4b3 marks

Use a genetic diagram to show the probability of the next child born to parents 5 and 6 of having muscular dystrophy.  

4c3 marks

Males are more likely than females to express the phenotype for muscular dystrophy.

Explain why. 

4d2 marks

Individual 3 and a heterozygous female had a female child who had muscular dystrophy.

Explain how this could happen.

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5a2 marks

Drosophila melanogaster is a fruit fly and is a useful organism to study patterns of inheritance. Female fruit flies can lay up to 400 eggs, developing into adults between 7 and 14 days. They have simple nutrient requirements.


Using the information provided explain two reasons why Drosophila melanogaster is a useful organism to study patterns of inheritance.

5b1 mark

In Drosophila melanogaster the genes for eye colour and antenna shape are linked.

Explain what this means. 

5c4 marks

Scientists investigated patterns of inheritance of eye colour and antenna shape in fruit flies. They carried out multiple crosses between fruit flies homozygous for both red eyes and bent antennae, and fruit flies homozygous for both white eyes and straight antennae. They found that all of the offspring produced in the F1 generation were heterozygous for both characteristics, having red eyes and bent antennae.

  • R represents the dominant allele for red eyes and r represents the recessive allele for white eyes
  • B represents the dominant allele for bent antenna and b represents recessive allele for straight antennae

The offspring from the F1 generation were crossed with fruit flies homozygous for both white eyes and straight antennae.

The results of these crosses are shown below in Table 1

Table 1

 

Red eyes, bent antennae

White eyes, straight antennae

Red eyes, straight antennae

White eyes, bent antennae

Number of offspring

488

481

93

97

Table 1 shows the number of offspring produced for each phenotype

Explain the results from Table 1.

5d2 marks

To determine if there is a significant difference between the observed results and expected results a statistical test can be used. State which statistical test should be used and explain your reasoning.

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1a2 marks

Aubergine (Solanum melongena L.) belongs to the same family as potatoes and tomatoes. They are a widely grown vegetable crop across Southeast Asia, Africa and the Mediterranean. Due to their importance as a food crop, scientists have been studying the inheritance of two genes (stem prickliness and fruit shape) that can have an influence on the profitability of the crop.  

The scientists examined 3000 offspring produced from the crosses between parent plants heterozygous for both genes. The allele P, for a non-prickly stem, is dominant to the allele p, for a prickly stem. The allele R, for round fruit, is dominant to the allele r, for linear fruit. 

Complete Table 1 below to show the corresponding phenotype or genotype.

Table 1

Phenotype of offspring

Genotype of offspring

Non-prickly stem, linear fruit

 
 

PpRR

 

ppRr

Prickly stem, linear fruit

 

1b4 marks

Complete Table 2 to show the expected ratio for each phenotype if the genes were located on different homologous pairs of chromosomes.

Table 2

Phenotype of offspring

Ratio of offspring

 
1c2 marks

In Table 3 are the phenotypes and number of offspring observed by the scientists.  

Table 3

Phenotype of offspring

Number of offspring

Non-prickly stem, round fruit

2175

Prickly stem, round fruit;

79

Non-prickly stem, linear fruit

67

Prickly stem, linear fruit

679

Suggest two reasons why the observed ratios do not reflect the scientists expected ratios.

1d2 marks

Draw a pair of chromosomes below to show the genotype that would obtain the observed numbers of offspring for each of the phenotypic ratios in Table 3.

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2a4 marks

In summer squash, two genes (A and B) interact to form the colour of the fruit. Figure 1 shows a drawing of a summer squash.

         Figure 1

jLpk8jQt_1          Summer Squash

Gene A controls whether the fruit has pigmentation or not. The dominant allele of this gene, A, results in no pigmentation, the squash are white. The recessive allele, a, results in pigmentation to the fruit. Gene B controls which pigmentation (yellow or green) the fruit has. The dominant allele, B, causes the fruit to have yellow pigmentation and the recessive allele, b, results in the fruit having green pigmentation. This gene has no effect on squash that have no pigmentation; white squash do not have any yellow or green pigmentation, even if they have the dominant B allele.

Scientists performed 4000 crosses in which white squash, heterozygous for both genes were crossed with yellow squash, heterozygous for gene B. They expected white, yellow and green squash in the offspring in a 4 : 3 : 1 phenotypic ratio.

Complete the genetic diagram to show how this ratio of phenotypes would be produced.

Parental phenotypes

White

Yellow

 

Parental genotypes

   
 

Gamete genotypes

   
 

Offspring genotypes

 
 

Offspring phenotypes

 

2b2 marks

The actual numbers of offspring with each phenotype were

White

1985

Yellow

1537

Green

478


The scientists carried out a  straight chi squared test on the data collected. Suggest the null hypothesis that was tested. 

2c2 marks

Complete the table to calculate the value of 2for these results.

Colour of offspring

Observed (O)

Expected (E)

open parentheses O minus E close parentheses to the power of blank

open parentheses O minus E close parentheses squared

open parentheses O minus E close parentheses squared over E

White

1985

       

Yellow

1537

       

Green

478

       
 

stack sum open parentheses O minus E to the power of blank close parentheses squared over E with blank below space equals

2d3 marks

The table shows values for chi2 at different levels of probability and for different degrees of freedom.

Degrees of freedom

Probability, p

0.2

0.1

0.05

0.02

0.01

1

1.64

2.71

3.84

5.41

6.64

2

3.22

4.61

5.99

7.82

9.21

3

4.64

6.25

7.82

9.84

11.35

4

5.99

7.78

9.49

11.67

13.28

5

7.29

9.24

11.07

13.39

15.09


State the conclusion the scientists should make about the significance of their results and explain your answer. 

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3a2 marks

Hypophosphatemia is a sex-linked inherited condition which results in abnormally low levels of phosphate in the blood which can cause the disease rickets. It is caused by a dominant allele.

Figure 1 shows the inheritance of hypophosphatemia in one family.

Figure 1ZY5ibdcp_2

State the evidence from Figure 1 that suggests that hypophosphatemia is a sex-linked, dominant inherited disease.

3b2 marks

Using the following symbols,

XH = an X chromosome carrying the allele for hypophosphatemia 

Xh = an X chromosome carrying the normal allele

Y = a Y chromosome

Give all the possible genotypes of each of the following persons,

 1  :

 4  :

 5  :

13 :

3c2 marks

Person 20 is pregnant for the fourth time. As the family has a history of hypophosphatemia, a test was carried out to discover the sex of the embryo.

Explain how observation of the chromosomes from an embryo cell could enable the sex to be determined.

3d4 marks

State the probability that the child Person 20 is pregnant with will be a male with hypophosphatemia. Explain your answer by drawing a genetic diagram, using the following symbols

XH = an X chromosome carrying the allele for hypophosphatemia 

Xh = an X chromosome carrying the normal allele

Y = a Y chromosome

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4a2 marks

A horticulturist investigated the monohybrid inheritance of flower colour in snapdragons. Two snapdragons with pink flowers were crossed. Of the offspring, 230 had pink flowers, 86 had red flowers and 84 had white flowers.

Using suitable symbols, give the genotypes of the parents. Explain your answer.

4b2 marks

When the horticulturist undertook the monohybrid cross, they had observed pink snapdragon flowers and white snapdragon flowers in their greenhouses. Therefore the ratio of flower colours obtained was unexpected. State and explain the expected ratio.

4c2 marks

As the horticulturist discovered the snapdragons were codominant for the flower colour gene, the phenotypic ratio was expected to be 1 white : 2 pink : 1 red. Suggest two reasons why the actual ratios observed are not the same as the expected ratios.

4d2 marks

To determine if the results obtained occurred due to chance the horticulturist undertook a statistical test. State which test was used and give the null hypothesis.

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5a2 marks

When most people cross their legs, they either always have their right leg on top, R, or always have their left leg on top, L. A scientist investigated this characteristic on four small islands, M, N, O and P. Their results are shown in Figure 1.

Figure 1

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On one of the islands, they recorded the leg-folding characteristics of parents and
their children. These results are shown in Table 1.

 Table 1

Leg-folding of parents

Leg-folding of the children/ %

 

Left leg on top, L

Right leg on top, R

L and L

56

44

R and R

53

47

R and L

58

42

The scientist concluded that leg-folding is not determined by a single gene with a dominant allele and a recessive allele.
Using information from Figure 1, describe the results the scientist obtained.

5b2 marks

Suggest why it was advantageous for the scientist to use island populations in this investigation.

5c3 marks

The scientist concluded that leg-folding is not determined by a single gene with a dominant allele and a recessive allele.

Use information from Table 1 to explain why they reached this conclusion.

5d1 mark

In another study, the scientist investigated leg-folding in genetically identical twins. Data from this study supported their conclusion from the island study.

Suggest the evidence they found that supported their conclusion.

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