AQA A Level Biology Paper 3 2021 Question Paper

AQA A Level Biology Paper 3 2021 Question Paper


Section A box

Answer all questions in this section.

You are advised to spend no more than 1 hour and 15 minutes on this section.

0 1 In one species of squirrel, Sciurus carolinensis, fur colour is controlled by one gene,

with two codominant alleles. CG

represents the allele for grey fur colour, and CB

represents the allele for black fur colour.

Table 1 shows the three possible phenotypes.

Table 1

Genotype Phenotype

CG

CG Grey fur

CG

CB Brown-black fur

CB

CB Black fur

0 1 . 1 In a population of 34 S. carolinensis, 2 had black fur.

Use the Hardy–Weinberg equation to estimate how many squirrels in this population

had brown-black fur. Show your working.

[2 marks]

Answer 

3

*03*

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outside the

0 1 box . 2 The actual number of squirrels in this population that had brown-black fur was 16.

Use all of the information to calculate the actual frequency of the CG

allele.

Do not use the Hardy–Weinberg equation in your calculation.

Give your answer to 2 decimal places.

[1 mark]

Answer

0 1 . 3 S. carolinensis were first introduced to the UK from North America in the 1870s.

They are now widely distributed across the UK.

S. carolinensis from both North America and the UK show exactly the same genotypic

and phenotypic variation. An identical mutation causing black fur has also been found

in several other species closely related to S. carolinensis.

Use this information to deduce which one of the following conclusions is most likely

true.

Tick (✓) one box.

[1 mark]

A The mutation that caused black fur happened after

S. carolinensis was introduced to the UK from North America.

B The mutation that caused black fur happened in a common

ancestor of S. carolinensis and other closely related species.

C The mutation that caused black fur happened independently

in S. carolinensis and all other closely related species.

D The phenotypic variation shown in S. carolinensis and other

closely related species is caused by genetic drift.

Question 1 continues on the next page

4

*04*

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Do not write

outside the

box The mutation that caused the CB

allele was due to a 24 base-pair deletion from the

CG

allele.

0 1 . 4 The protein coded for by the CB

allele is 306 amino acids long.

Calculate the percentage reduction in size of the protein coded for by the CB

allele

compared with the protein coded for by the CG

allele.

Give your answer to 3 significant figures and show your working.

[2 marks]

Answer

In S. carolinensis, fur colour depends on the distribution and relative amounts of light

pigments and dark pigments in the hairs of the fur. Figure 1 shows how the protein

produced from the CG

allele can result in the production of a light pigment or a dark

pigment.

Figure 1

5

*05*

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outside the

box The deletion mutation in the CB

allele results in the production of a receptor protein

that does not have glutamic acid. The lack of glutamic acid in the receptor protein has

the same effect as αMSH leaving the receptor protein.

0 1 . 5 Use Figure 1 and this information to suggest why S. carolinensis with the genotype

CB

CB

have black fur rather than grey fur.

[3 marks]

Turn over for the next question

9

6

*06*

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Do not write

outside the

box 0 2 . 1 Describe how the human immunodeficiency virus (HIV) is replicated once inside

helper T cells (TH cells).

[4 marks]

7

*07*

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outside the

HIV-1 is the most common type of HIV. HIV-1 binds to a receptor on TH cells called box

CCR5.

Current treatment for HIV-1 involves the use of daily antiretroviral therapy (ART) to

stop the virus being replicated. Only 59% of HIV-positive individuals have access to

ART.

Scientists have found that two HIV-1-positive patients (P and Q) have gone into

remission (have no detectable HIV-1). This happened after a blood stem cell

transplant (BSCT).

• Patient P was given two BSCTs, and patient Q was given one BSCT.

• All BSCTs came from a donor with TH cells without the CCR5 receptor.

• In addition, patient P had radiotherapy, and patient Q had chemotherapy. Both of

these treatments are toxic.

• Both patients (P and Q) stopped receiving ART 16 months after BSCT.

18 months after stopping ART, both patients had no HIV-1 RNA in their plasma,

no HIV-1 DNA in their TH cells and no CCR5 on their TH cells.

0 2 . 2 Use the information given to evaluate the use of BSCT to treat HIV infections.

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Category AQA Questions and Marking Scheme
Authors Qwivy.com
Pages 37
Language English
Tags AQA A Level Biology Paper 3 2021 Question Paper
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