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*JUN217402301*

IB/H/Jun21/E14 7402/3

For Examiner’s Use

Question Mark

1

2

3

4

5

6

7

TOTAL

Time allowed: 2 hours

Materials

For this paper you must have:

• a ruler with millimetre measurements

• a scientific calculator.

Instructions

• Use black ink or black ball-point pen.

• Fill in the boxes at the top of this page.

• Answer all questions in Section A.

• Answer one question from Section B.

• You must answer the questions in the spaces provided. Do not write

outside the box around each page or on blank pages.

• If you need extra space for your answer(s), use the lined pages at the end of

this book. Write the question number against your answer(s).

• Show all your working.

• Do all rough work in this book. Cross through any work you do not want

to be marked.

Information

• The marks for the questions are shown in brackets.

• The maximum mark for this paper is 78.

Please write clearly in block capitals.

Centre number Candidate number

Surname

Forename(s)

Candidate signature

I declare this is my own work.

A-level

BIOLOGY

Paper 3

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IB/H/Jun21/7402/3

Do not write

outside the

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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3

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IB/H/Jun21/7402/3

Do not write

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