AQA A-level biology. Student guide 4, Topics 7 and 8

83 Pages·2016·23.137 MB·English

by  Pauline Lowrie

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A-LEVEL YEAR 2
STUDENT GUIDE
AQA 
Biology
Topics 7 and 8
Genetics, populations, evolution and
ecosystems
The control of gene expression
Pauline Lowrie
The author would like to thank South Sefton Sixth Form College.
Philip Allan, an imprint of Hodder Education, an Hachette UK company, Blenheim Court,
George Street, Banbury, Oxfordshire OX16 5BH
Orders
Bookpoint Ltd, 130 Park Road, Milton Park, Abingdon, Oxfordshire OX14 4SE
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Lines are open 9.00 a.m.–5.00 p.m., Monday to Saturday, with a 24-hour message answering
service. You can also order through the Hodder Education website: www.hoddereducation.co.uk
© Pauline Lowrie 2016
ISBN 978-1-4718-5838-3
First printed 2016
Impression number 5 4 3 2 1
Year 2020 2019 2018 2017 2016
All rights reserved; no part of this publication may be reproduced, stored in a retrieval system,
or transmitted, in any other form or by any means, electronic, mechanical, photocopying,
recording or otherwise without either the prior written permission of Hodder Education
or a licence permitting restricted copying in the United Kingdom issued by the Copyright
Licensing Agency Ltd, Saffron House, 6–10 Kirby Street, London EC1N 8TS.
This guide has been written specifically to support students preparing for the AQA A-level
Biology examinations. The content has been neither approved nor endorsed by AQA and
remains the sole responsibility of the author.
Cover photo: Sergey Nivens/Fotolia
Typeset by Integra Software Services Pvt. Ltd, Pondicherry, India
Printed in Italy
Hachette UK’s policy is to use papers that are natural, renewable and recyclable products and
made from wood grown in sustainable forests. The logging and manufacturing processes are
expected to conform to the environmental regulations of the country of origin.
Contents
Getting the most from this book . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
About this book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Content Guidance
Genetics, populations, evolution and ecosystems . . . . . . . . . . . . . 7
Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Populations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Evolution may lead to speciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Populations in ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Required practical 12: Investigating the effect of environmental
factors on the distribution of a species . . . . . . . . . . . . . . . . . . . . . . . .26
The control of gene expression . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Alteration of the sequence of bases in DNA can alter
the structure of proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Gene expression is controlled by a number of features . . . . . . . . . .28
Using genome projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
Gene technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36
Questions & Answers
Paper 2-type questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Paper 3-type questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Required practical 12 answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Knowledge check answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
■ Getting the most from this book
Exam tips
Knowledge check
Advice on key points in the text to
help you learn and recall content, Rapid-fi re questions throughout
avoid pitfalls, and polish your exam the Content Guidance section to
technique in order to boost your check your understanding .
grade .
Summaries
Knowledge check answers
■ Each core topic is rounded
 1 Turn to the back of the book
off by a bullet-list summary
for the Knowledge check
for quick-check reference of
answers.
what you need to know .
Questions & Answers
Student B
(c) Yes because this animal will be cheaper to feed as it digests more of its
food ✓ and it causes less pollution. The farmer will make more profit.
However, genetic engineering is unnatural and scientists should not be
altering the genetics of pigs.
eThis gets only 1 mark. Although the student understands the pig will cause
less pollution, this is unspecific. The student does not explain why the pig will
be more profitable. The argument against genetic engineering is not made
scientifically. The student should explain a concern about genetic engineering in a
scientific way (as student A did) and/or explain the concerns that consumers have
which might affect the farmer’s ability to sell his livestock. Commentary on sample
Question 3
Exam-style questions Palhlleolxe sis. Pa ltaynptes ohfo gmaordzeygno fluosw foerr. S F1l howaveer bsrhoaapde fliso wcoenrtsr.o Pllleadn tbsy h towmo oczoydgoomuisn faonrt student answers
S2 have ‘cuspidata’ flowers, and heterozygous plants have ‘fimbriata’ flowers.
Aalnleolteh eFr f goer nflea tc opnettraolsls i sw dhoemthienra ntht eo vpeert atlhse l aiel lflelaet fo fro fro frumn nae flu-nshnaepl eshda ppeet.a Tlsh.e Find out how many marks
(a) What are codominant alleles? (AO1) (1 mark)
each answer would be
Student A
(a) Tbohtehs eh oamreo azlylgeoletess t.hat produce a different phenotype in a heterozygote from awarded in the exam and then
eA fully correct answer gaining the mark. read the comments (preceded
Student B
(a) This is when one allele is not dominant over the other. by the icon e) following
Sample student beut Tdhoiess g naoint se xnpol amina rwkhsa. tT iht iiss .t eItl lcsa yno bue o dnief faiclluellte t ois e nxoptl adionm thinea mnte oavneinr gth oef ogtehneert,i c each student answer showing
terms clearly, so practise this before the exam.
answers (b) Twweore p claronstss,e bdo. tUhs we iath g fiemnebtricia dtiaa flgroawme rtso asnhdo whe ttheer orazytigoo oufs p fhoern poettyaple ssh aanpde , exactly how and where marks
genotypes expected in the offspring. (AO2) (4 marks) are gained or lost.
Practise the questions, then Student A
(b) Parent phenotypes fimbriata, flat × fimbriata, flat
look at the student answers Parent genotypes S1S2Ff × S1S2Ff ✓
Gametes S1F S1f S2F S2f × S1F S1f S2F S2f ✓
that follow.
54 AQA Biology
4  AQA Biology
■ About this book
This guide will help you to prepare for topics 7 and 8. These topics are examined in
paper 2 (together with topics 5 and 6) and in paper 3 (together with topics 1–4 and 5–6).
The Content Guidance covers all the facts you need to know and concepts you need
to understand for topics 7 and 8. It is important that you focus on understanding and
not just learning facts, as the examiners will be testing your ability to apply what you
have learned in new contexts. This is impossible to do unless you really understand
everything. The Content Guidance also includes exam tips and knowledge checks to
help you prepare for your exams.
The Questions & Answers section shows you the sorts of questions you can expect
in papers 2 and 3. It would be impossible to give examples of every kind of question in
one book, but these should give you a flavour of what to expect. Two students, student
A and student B, attempt each question. Their answers, and the accompanying
comments, should help you to see what you need to do to score a good mark — and
how you can easily not score a mark even though you probably understand the biology.
What can I assume about the guide?
You can assume that:
■ the basic facts you need to know and understand are stated explicitly
■ the major concepts you need to understand are explained clearly
■ the questions at the end of the guide are similar in style to those that will appear in
the fi nal examination
■ the questions assess the different assessment objectives
■ the standard of the marking is broadly equivalent to that which will be applied to
your answers
How should I use this guide?
The guide lends itself to a number of uses throughout your course — it is not just a
revision aid. You could:
■ use it to check that your notes cover the material required by the specifi cation
■ use it to identify your strengths and weaknesses
■ use it as a reference for homework and internal tests
■ use it during your revision to prepare ‘bite-sized’ chunks of related material, rather
than being faced with a fi le full of notes
You could use the Questions & Answers section to:
■ identify the terms used by examiners and show what they expect of you
■ familiarise yourself with the style of questions you can expect
■ identify the ways in which students gain, or fail to gain, marks
Topics 7 and 8 5
About this book
Develop your examination strategy
Just as reading the Highway Code alone will not help you to pass your driving test, this
guide cannot help to make you a good examination candidate unless you develop and
maintain all the skills that examiners will test in the final exams. You also need to be
aware of the type of questions examiners ask and where to find them in the exams.
You can then develop your own personal examination strategy. But, be warned, this is
a highly personal and long-term process; you cannot do it a few days before the exam.
Things you must do
■ Clearly, you must know some biology. If you do not, you cannot expect to get a
good grade. This guide provides a succinct summary of the biology you must know.
■ Be aware of the skills that examiners must test in the exams. These are called
assessment objectives and are described in the AQA Biology specifi cation.
■ Understand the weighting of the assessment objectives that will be used. These are
as follows:
Assessment  Brief summary Marks in  Marks in  Marks in 
objective A-level  A-level  A-level 
paper 1/% paper 2/% paper 3/%
AO1 Knowledge and 44–48 23–27 28–32
understanding
AO2 Application of 30–34 52–56 35–39
knowledge and
understanding
AO3 Analyse, interpret 20–24 19–23 31–35
and evaluate
scientific
information, ideas
and evidence
■ Use past questions and other exercises to develop all the skills that examiners must
test. Once you have developed them all, keep practising to maintain them.
■ Understand where in your exams different types of questions occur. For example,
the fi nal question on A-level paper 3 will always be worth 25 marks and will test
AO1 by requiring you to write an essay. If that is the skill in which you feel most
comfortable, and many A-level students do, why not attempt this question fi rst?
■ Remember that mathematical skills account for about 10% of the marks. Do make
sure you can carry out these calculations, including percentages, ratios, and rates
of reaction.
■ You need to be familiar with the techniques you have learned in the required
practicals, and be able to describe how these techniques might be used in a
different context. Also, you need to be able to evaluate practical investigations and
data presented to you in the exam.
6  AQA Biology
Content Guidance
■ Genetics, populations,
evolution and ecosystems
Inheritance
You will remember from the first year of your course that a gene is a length of DNA at
a specific place on a chromosome, or locus, that codes for a polypeptide. There may
be different versions of the same gene, called alleles, which code for slightly different
polypeptides. Diploid organisms have two copies of each chromosome in their body
cells, so they have two alleles of each gene in each cell. These alleles may be both the
same (homozygous) or different (heterozygous).
The genetic make-up of an organism is called its genotype. The phenotype is
the appearance of the organism resulting from the expression of its genes and its
interaction with the environment.
Knowledge check 1
Match the words with their defi nition .
1 Locus A A piece of DNA that codes for a polypeptide
2 Heterozygote B A specifi c place on a chromosome where a gene
is found
3 Homozygote C The genetic make-up of an organism
4 Gene D A different version of a gene
5 Genotype E An individual with two different alleles of a gene
6 Phenotype F An organism with two copies of each
chromosome in their body cells
7 Allele G The appearance of an organism resulting from
the expression of its genes and its interaction
with the environment
8 Diploid H An individual with two identical alleles of a gene
Monohybrid crosses
Monohybrid crosses involve a single pair of alleles. In the simplest situations, one of
the alleles is dominant and the other is recessive. A dominant allele is expressed in
the phenotype even if only one copy is present. A recessive allele is not expressed in
the phenotype unless two copies are present. We use a specific letter for one gene.
We use the upper case letter to represent the dominant allele, and the lower case
letter to represent the recessive allele.
Topics 7 and 8 7
Content Guidance
It is important that you set out your working clearly in a genetics problem. Exam tip
For example:
If you choose your own
In dogs, wire hair is dominant over smooth hair. A breeder crosses a heterozygous symbols for a genetics
wire-haired animal with a smooth-haired animal. What proportions of hair type would problem, it is better to
you expect in the offspring? choose a letter where
the upper case and
Parent phenotype wire haired smooth haired
lower case letters are
Parent genotype Hh hh
different in shape . Here
Gametes H h × h h H (for hair) was chosen
as H and h are different
Offspring genotypes
shapes . If we had used
S or W, and writing the
H h
letters by hand in an
h Hh hh
exam, it would be easy
h Hh hh
to confuse an upper
case letter for a lower
Offspring phenotypes wire haired : smooth haired
case letter, or vice
2 : 2 versa .
Answer: half the offspring will have wire hair and the other half will have smooth hair.
Exam tip Knowledge check 2
There is a sequence of stages to carry out in any genetics problem . The allele for cystic
fi brosis is recessive to
1 Work out the parent genotypes .
the normal allele . Two
2 Work out the gametes produced .
parents who are both
3 Set up a Punnett square with the gametes of one parent along the top
normal have a baby
and the gametes of the other parent on the side .
with cystic fi brosis .
4 Fill in the table with the genotypes of the offspring, by pairing the
What is the chance
gamete at the top of the row with the gamete at the side .
that a second baby
5 Find the ratio of genotypes in the offspring .
might also have cystic
6 Work out the ratio of phenotypes in the offspring . fi brosis?
7 Answer the question you were asked .
However, some alleles do not show a straightforward dominant–recessive relationship.
Alleles may be codominant. Codominance occurs where the heterozygote has a
phenotype that is different from both homozygotes. We use different nomenclature
for codominance. There is a capital letter to represent the gene, with the alleles in
upper case letters in superscript. An example is shown below.
In shorthorn cattle, red coat colour is codominant with white coat colour. A mating
between a red individual and a white individual can produce offspring that are roan,
with a mixture of red and white hairs. Use a Punnett square to predict the ratio of
phenotypes you would expect from a mating between a roan cow and a white bull.
We will use the symbol CR for the red allele and CW for the white allele. We know
that the white parent is homozygous, since a heterozygote has a different phenotype
from both homozygotes. The roan parent must be heterozygous. So we can set out the
problem similarly to the last example.
8  AQA Biology