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
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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
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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
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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
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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]
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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.
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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