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Section E: Variation and Selection
Chapter 16: Chromosomes,genes and DNA
1 parents offspring genes chromosomes characteristics
2 nucleus This contains the chromosomes carrying thousands of genes.
sperm Its nucleus contains the chromosomes that carry genes from the father.
egg Its nucleus contains the chromosomes that carry genes from the mother.
fertilised egg Its nucleus contains chromosomes from both parents.
3 gene a unit of genetic information that is linked to a particular characteristic. It
controls production of proteins in the cell.
alleles different forms of a gene
dominant allele an allele that controls the development of a characteristic even when it is
present on only one of the chromosomes
recessive allele an allele that controls the development of a characteristic only if it is present on
both chromosomes
4 a) By the inheritance of the sex chromosomes X and Y. Females inherit XX males XY.
b) i! "#
ii! Because a Barr body in the cell exists when one of two X chromosomes is deactivated and
only women have two X chromosomes so only women have Barr bodies in their cells.
5 mutations genes variety radiation mutagens
6 a) To protect them from the radiation which could cause mutations
b) Because it would be easy for them to be exposed to high levels of radiation which could cause
mutations in their gonads or gametes. These would be passed on to their children where the
effects might then show up as genetic disorders or cancers.
7 a) $moking increases your risk of dying from lung cancer.
b) $moking increases the levels of these chemicals in your body.c) Because cigarette smoke is taken into the lungs so the concentration of carcinogenic chemicals
is particularly high in that tissue.
d) In the throat and mouth because the smoke also passes across these tissues every time it is
inhaled.
8 a) %raph of the data
b) It shows that the rate of mutation increases as the dose of radiation goes up.
c) Because increased radiation increases the probability of mutation which in turn tends to cause
an increase in cancer rates. The exposure is monitored to try to reduce the health risk to people
working with radiation.
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9 a) &utations in the reproductive cells can lead to infertility or to the birth of babies suffering from
genetic abnormalities.
b) &utations in normal body cells might increase the risk of cancers developing.
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Chapter 17: Cell division
1
In aseual re!roduction no cells "oin and the new individual is identical to the !arent#
In seual re!roduction s!ecial male and $emale se
cells $use %"oin&
to $orm a uni'ue new cell#
(he new individual $ormed in seual re!roduction contains a miture o$ genetic in$ormation $rom )oth
!arents#
(he s!ecial se cells involved in seual
re!roduction
are *nown as gametes#
A clone is the identical o$$s!ring $ormed as a result o$ aseual re!roduction#
2 a)
b) $o that the new cells are exactly the same as the old ones so that the new individual is made up
of the same uni'ue cells and so that the cells all carry the right information to enable them to do
their (obs.
c) It is the way the body makes new cells to replace old worn)out ones and to heal damaged tissue.
3 a) #*
b) "+
c) &eiosis
d) In the ovaries and testes
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e)
4 a) In human reproduction meiosis is important in the formation of the sex cells. It halves the
number of chromosomes so when the sex cells (oin a normal cell with #* chromosomes results
and it introduces variety. ,fter fertilisation mitosis is important in the formation of the millionsof cells needed to make up the new individual.
b) &itosis is important for making the new cells needed to give the cuttings roots and then for
normal growth to continue. &eiosis plays no part.
5 a) -orrectly draw bar graphs
b) eight
c) Because their height varied relatively little whether the twins were brought up together or apart
whereas their mass was similar when brought up together and fed the same food but much less
similar when brought up apart.
d) -an give credit for any answer well supported by the explanation. owever the expected
answer might be on the following lines.
The information is difficult to collect mainly because identical twins themselves are relatively
rare so identical twins who are separated and brought up apart for long periods of time meeting
up later to enable comparisons to be made are very rare indeed.
/on)identical twins and non)twin siblings are not particularly similar in height and there is a
big difference between them and identical twins however the twins are brought up. owever
identical twins brought up apart show the same difference in mass as the other two groups
showing a much bigger influence of the environment on body mass.
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Chapter 18: Genes and inheritance
1 homo0ygous characteristic hetero0ygous dominant phenotype recessive
genotype codominant
2 a) $andy1s dimple alleles are dd. $he doesn1t have dimples so she must be homo0ygous recessive.
b) Tom1s dimple alleles would be 2d. e must have a recessive allele to pass on even though he
shows the dominant character trait.
c) They would be 2d or 22 because he has passed on a dominant allele.
3 a) rr 3 a homo0ygous recessive plant
b) i! 4arents 55 6 rr
%ametes 5 5 6 r r
5 5
r 5r 5r
r 5r 5r
7ffspring 5r 8all smooth and round!
ii! 4arents 5r 6 rr
%ametes 5 r 6 r r
5 rr 5r rr
r 5r rr
7ffspring 5r 5r 8smooth and round! or rr rr 8wrinkled!
4 a) 4arents Tt 6 tt
%ametes T t 6 t t
T t
t Tt tt
t Tt tt
7ffspring Tt Tt 8&anx! or tt tt 8normal!
b) 4arents Tt 6 Tt
%ametes T t 6 T t
T t
T TT tT
t Tt tt
7ffspring TT Tt tT tt
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The expected ratio is +9:.
c) The ratio of &anx kittens actually born is " &anx to : normal because the homo0ygous &anx
kittens die before birth.
5 a) F is the normal allele f is the cystic fibrosis gene.
4arents Ff 8Fran;ois! 6 FF 8,nnette!
%ametes Ff 6 FF
F f
F FF fF
F FF fF
4ossible offspring9 FF FF fF fF
There is no chance of their having a child with cystic fibrosis although some of their children
may be carriers of the faulty gene.
b) If neither has the faulty gene 8FF 6 FF! then all offspring FF
If only one has the faulty gene then as in part awith offspring FF or fF
If both are carriers 8Ff 6 Ff! then
F f
F FF fF
f Ff ff
4ossible offspring9 FF Ff fF ff giving a :9# chance of their producing a child with cystic
fibrosis.
6 a) 4upils should choose a letter that looks different in upper and lower case e.g. 2 for dwarfism
with d for normal which are used here.
Individual , has genotype 2d B has genotype dd and - has genotype 2d.
b) It could be that the embryo was homo0ygous dominant which is lethal9
4arents 2d 6 2d
%ametes 2d 6 2d
2 d
2 22 d2
d 2d dd
7ffspring9 22 8dies< maybe cause of miscarriage! 2d or d2 8achondroplastic dwarfs! dd
8normal height!
7 a) -odominance occurs when a gene has two alleles neither of which is dominant to the other and
both of which contribute to the phenotype.
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b) I would carry out crosses between the red 85! and white 8=! flowered plants. $everal different
outcomes are possible but we know that some of the offspring will be hetero0ygous. $o if there
is codominance there should be some pink flowers. There are two possible crosses9
5= 6 ==9
5 =
= 5= ==
= 5= ==
This gives " red and " white offspring so no codominance
55 6 ==9
5 5
= 5= 5=
= 5= 5=
If all offspring red no codominance. If all offspring pink there is codominance.
8 $ex is inherited on whole chromosomes 3 XX female XY male. $o a baby boy inherits the X
chromosome from his mother and the Y from his father. 2imples 82! are inherited on a single gene
with two different alleles so dimples are dominant. The baby has inherited an allele for dimples from
one parent or the other9
X X
X XX XXY XY XY
>arious combinations could result in the baby inheriting dimples e.g. 22 6 22 22 6 2d 22 6 dd
2d 6 22 2d 6 2d or 2d 6 dd. 4upils could give genetic diagrams for all these crosses.
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Chapter 19: Natral selection and evoltion
1 ,fter the first application of a pesticide the ma(ority of the insects are killed. &ost of the insects that
survive will stay alive because they are resistant in some way to the pesticide as the result of some
mutation that had occurred when they developed. These insects then continue to live and reproduceso that when the population is again exposed to the pesticide most of the insects will be resistant
descendants of the original survivors and so will be unaffected by the spray. $ome however will
have mutated again and lost their resistance so they will be killed by the pesticide.
2 a) orse
b) 1 The animal was small. It lived in swamps so it needed spread)out feet to avoid sinking. It
probably didn1t move very rapidly relying on camouflage and its ability to walk on soft
ground to escape predators.
2 The animal was still small and so probably depended on camouflage and disappearing in
the undergrowth for protection but as it was no longer a swamp dweller its foot was less
spread to make walking on dry forest floors and prairies easier.
3 $peed is becoming more important as an escape method as there is little cover for hiding.
7nly one point of the toe now touches the ground and the animal is developing longer legs
to go with the modified foot for running faster.
4 The shape of the modern horse begins to emerge. The leg and foot are modified so that
only a single toe remains and all the other toes are effectively lost into the legs which are
much longer. The angle of the (oints is changing and the hind 'uarters gain big muscles for
fast running on dry grasslands.
5 The animal is much taller with very long legs and single toes well evolved for fast running
on open grasslands both as part of a herd lifestyle and to escape predators. It can see over
long grass easily.
3 a) Because different genetic variations exist in the same population.
b) Yes 3 it would make the birds more obvious to predators.
c) Yes 3 birds with white feathers are more likely to survive the winter without being eaten and so
are more likely to survive and breed.
d) Yes 3 brown birds are mutations and are relatively unsuccessful so they stay as a minority in thepopulation.
4 a) Two years 3 to :?:@
b) :?+A or :?++ would be acceptable
c) /ot until :?+@ "" years from the first appearance of the disease
d) =hen the disease first appeared virtually all of the oysters were wiped out. 7f those that
remained some were lucky though they may have succumbed to the disease later< others must
have possessed a mutation that gave them immunity to the disease. This mutation allowed a tiny
number of oysters to survive and become a breeding colony which once established bred rapidlyto restore the oyster numbers with oysters that were now immune to the disease.
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5 The theory of natural selection says that individual organisms within a species may show a wide
range of variation because of differences in their genes. Individuals with characteristics most suited
to their environment are more likely to survive and breed successfully and the genes that have
enabled these individuals to survive are then passed on to the next generation. This explains the
situation with the %alpagos finches.
Birds arrived on a particular island or an area of an island from the mainland blown by a storm orsimilar. Those birds with beaks best suited to a particular food type 3 either one that was very
common in the area or something not exploited by other animals 3 would be most successful and
most likely to breed passing on the genes for the slightly modified beak. 7ver many generations this
effect would be magnified until the birds formed separate breeding colonies with separate feeding
strategies and different beak structures. Cach type of finch was particularly successful in its own
niche. Breeding isolation would be achieved by distance and also by changing breeding behaviour
8e.g. song displays! that was no longer universally recognised.
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Chapter 2!: Selective "reedin#
1 cuttings identical characteristics damp plastic bags
2 a) $elective breeding
b)
$ni%al selected &eason 'h( )lant selected &eason 'h(
hens $rom wild
chic*ens
la( %ore e##s wheat $rom wild grasses large +ears $or $ood
!igs $rom wild )oar ta%er* %ch %ore
%eat* #ro' +aster
!otatoes $rom wild
!otatoes
#ro' %ore* "i##er
potatoes
co's +ro% 'ild cattle large mil* !roduction,
lots o$ meat
an( #arden +rit* e,#,
apples* stra'"erries
larger, sweeter $ruit
dogs $rom wolves ta%er* s%aller* %ore
o"edient
garden roses $rom wild
roses
lar#er* %ore color+l
+lo'ers* stron#er scent
3 a) Tissue culture uses minute collections of cells as the starting point. Traditional cuttings uses
parts of whole stems and roots.
b) : Darge numbers of genetically identical plants 8clones! can be produced from (ust one plant.
" &icropropagation can produce large numbers of new plants that might be difficult to
produce from seeds or traditional cuttings.
+ /ew plants can be made all year by growing them in a laboratory.
# Darge numbers of plants can be stored easily.
c) There is no variety in the population so if one plant cannot cope with changed conditions none
of them will be able to cope and they will all die.
4 a) It is increasing at a very rapid rate.
b) Because we need to be able to feed all the people.
c) -orrectly drawn graph with labelled axes etc.
d) 4otatoes and citrus fruits
e) Because otherwise many of the extra crops grown will be lost to disease. By making crops
resistant to disease the yield can be increased.
5 a) -orrectly drawn graphs of yields with time labelled axes etc.
b) Because their milk yield was overtaken by other breeds so farmers stopped farming them.
c) There is a lot of cheap milk available for milk and milk products.
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d) The big increase in milk production is in areas such as Curope while the main need for extra
food is in the developing world. $o in Curope there is a Emilk lake1 3 more milk is being
produced than can be used.
e) The same sort of pattern with increasing yields over the last @A years
f) -ross two particularly large parents with a good meat yield and then cross the offspring again
with particularly large animals etc. ,void breeding from animals with lighter carcasses.
6 Flow diagram to follow this basic se'uence 8the first two points are relatively interchangeable!9
Take an adult cell from a mature animal with a diploid nucleus
Take an egg from another mature female of the same species and remove the nucleus
-ombine the adult cellnucleus with the empty egg
%ive a small electric shock to stimulate division
,llow the embryo to start dividing
5eplace the embryo in a foster mother of the same species
7ffspring eventually born is a clone of the original adult animal
7 a) Both allow large numbers of genetically identical individuals to be produced from good parent
stock much more 'uickly and reliably than would be possible using traditional techni'ues.
b) -loning plants uses bits of the adult plant as the raw material. -loning animals currently
involves using normal body cells and egg cells as the raw material though this may change in
the future 3 2olly needed an egg cell and the nucleus from a mammary gland cell.
c) There are more and more people in the world who need to be fed so there will always be a needfor techni'ues for reproducing high)yielding plants and animals. ,lso in developed countries
people demand high)'uality but inexpensive food so techni'ues that reproduce valuable
animals and plants more 'uickly are valued.
