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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) 24

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) Smoking increases your risk of dying from lung cancer.

b) Smoking 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) Graph 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) Mutations in the reproductive cells can lead to infertility or to the birth of babies suffering from genetic abnormalities.

b) Mutations in normal body cells might increase the risk of cancers developing.

Chapter 17: Cell division 1

In asexual reproduction no cells join and the new individual is identical to the parent.

In sexual reproduction special male and female sex cells fuse (join)

to form a unique new cell.

The new individual formed in sexual reproduction contains a mixture of genetic information from both parents.

The special sex cells involved in sexual reproduction

are known as gametes.

A clone is the identical offspring formed as a result of asexual reproduction.

2 a)

b) So that the new cells are exactly the same as the old ones, so that the new individual is made up of the same unique cells, and so that the cells all carry the right information to enable them to do their jobs.

c) It is the way the body makes new cells to replace old worn-out ones and to heal damaged tissue.

3 a) 46

b) 23

c) Meiosis

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 join, a normal cell with 46 chromosomes results, and it introduces variety. After fertilisation, mitosis is important in the formation of the millions of cells needed to make up the new individual.

b) Mitosis is important for making the new cells needed to give the cuttings roots, and then for normal growth to continue. Meiosis plays no part.

5 a) Correctly draw bar graphs

b) Height

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) Can give credit for any answer well supported by the explanation. However, 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.

Non-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. However, 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 homozygous characteristic heterozygous dominant phenotype recessive

genotype codominant

2 a) Sandys dimple alleles are dd. She doesnt have dimples, so she must be homozygous recessive.

b) Toms dimple alleles would be Dd. He must have a recessive allele to pass on, even though he shows the dominant character trait.

c) They would be Dd or DD, because he has passed on a dominant allele.

3 a) rr a homozygous recessive plant b) i) Parents RR rr Gametes R R r r

R R

r Rr Rr

r Rr Rr

Offspring Rr (all smooth and round)

ii) Parents Rr rr Gametes R r r r

R r

r Rr rr

r Rr rr

Offspring Rr Rr (smooth and round) or rr rr (wrinkled)

4 a) Parents Tt tt Gametes T t t t

T t

t Tt tt

t Tt tt

Offspring Tt Tt (Manx) or tt tt (normal)

b) Parents Tt Tt Gametes T t T t

T t

T TT tT

t Tt tt

Offspring TT Tt tT tt

The expected ratio is 3:1.

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c) The ratio of Manx kittens actually born is 2 Manx to 1 normal, because the homozygous Manx kittens die before birth.

5 a) F is the normal allele, f is the cystic fibrosis gene.

Parents Ff (Franois) FF (Annette) Gametes Ff FF

F f

F FF fF

F FF fF

Possible offspring: 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 (FF FF), then all offspring FF

If only one has the faulty gene, then as in part a with offspring FF or fF

If both are carriers (Ff Ff), then

F f

F FF fF

f Ff ff

Possible offspring: FF Ff fF ff, giving a 1:4 chance of their producing a child with cystic fibrosis.

6 a) Pupils should choose a letter that looks different in upper and lower case, e.g. D for dwarfism, with d for normal, which are used here.

Individual A has genotype Dd, B has genotype dd, and C has genotype Dd.

b) It could be that the embryo was homozygous dominant, which is lethal:

Parents Dd Dd Gametes Dd Dd

D d

D DD dD

d Dd dd

Offspring: DD (dies; maybe cause of miscarriage), Dd or dD (achondroplastic dwarfs), dd (normal height)

7 a) Codominance 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 (R) and white (W) flowered plants. Several different outcomes are possible, but we know that some of the offspring will be heterozygous. So if there is codominance, there should be some pink flowers. There are two possible crosses:

RW WW:

R W

W RW WW

W RW WW

This gives 2 red and 2 white offspring, so no codominance

RR WW:

R R

W RW RW

W RW RW

If all offspring red, no codominance. If all offspring pink, there is codominance.

8 Sex is inherited on whole chromosomes XX female, XY male. So a baby boy inherits the X chromosome from his mother and the Y from his father. Dimples (D) 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 other:

X X

X XX XX

Y XY XY

Various combinations could result in the baby inheriting dimples, e.g. DD DD, DD Dd, DD dd, Dd DD, Dd Dd, or Dd dd. Pupils could give genetic diagrams for all these crosses.

Chapter 19: Natural selection and evolution 1 After the first application of a pesticide, the majority of the insects are killed. Most 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 reproduce, so 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. Some, however, will have mutated again and lost their resistance, so they will be killed by the pesticide.

2 a) Horse

b) 1 The animal was small. It lived in swamps, so it needed spread-out feet to avoid sinking. It probably didnt 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.

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3 Speed is becoming more important as an escape method, as there is little cover for hiding. Only 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 joints is changing and the hind quarters 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 it would make the birds more obvious to predators.

c) Yes 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 brown birds are mutations and are relatively unsuccessful, so they stay as a minority in the population.

4 a) Two years to 1917

b) 1930 or 1933 would be acceptable

c) Not until 1937, 22 years from the first appearance of the disease

d) When the disease first appeared, virtually all of the oysters were wiped out. Of 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 rapidly to restore the oyster numbers with oysters that were now immune to the disease.

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 Galpagos finches.

Birds arrived on a particular island or an area of an island from the mainland, blown by a storm or similar. Those birds with beaks best suited to a particular food type either one that was very common in the area or something not exploited by other animals would be most successful and most likely to breed, passing on the genes for the slightly modified beak. Over many generations this effect would be magnified, until the birds formed separate breeding colonies with separate feeding strategies and different beak structures. Each type of finch was particularly successful in its own niche. Breeding isolation would be achieved by distance and also by changing breeding behaviour (e.g. song, displays) that was no longer universally recognised.

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Chapter 20: Selective breeding 1 cuttings identical characteristics damp plastic bags

2 a) Selective breeding

b)

Animal selected Reason why Plant selected Reason why

hens from wild chickens

lay more eggs wheat from wild grasses

large ears for food

pigs from wild boar tamer, much more meat, grow faster

potatoes from wild potatoes

grow more, bigger potatoes

cows from wild cattle large milk production, lots of meat

any garden fruit, e.g. apples, strawberries

larger, sweeter fruit

dogs from wolves tamer, smaller, more obedient

garden roses from wild roses

larger, more colourful flowers, stronger scent

3 a) Tissue culture uses minute collections of cells as the starting point. Traditional cuttings uses parts of whole stems and roots.

b) 1 Large numbers of genetically identical plants (clones) can be produced from just one plant.

2 Micropropagation can produce large numbers of new plants that might be difficult to produce from seeds or traditional cuttings.

3 New plants can be made all year by growing them in a laboratory.

4 Large 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) Correctly drawn graph with labelled axes, etc.

d) Potatoes 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) Correctly 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 Europe, while the main need for extra food is in the developing world. So in Europe there is a milk lake more milk is being produced than can be used.

e) The same sort of pattern, with increasing yields over the last 70 years

f) Cross two particularly large parents with a good meat yield, and then cross the offspring again with particularly large animals, etc. Avoid breeding from animals with lighter carcasses.

6 Flow diagram to follow this basic sequence (the first two points are relatively interchangeable):

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

Combine the adult cell/nucleus with the empty egg

Give a small electric shock to stimulate division

Allow the embryo to start dividing

Replace the embryo in a foster mother of the same species

Offspring 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 quickly and reliably than would be possible using traditional techniques.

b) Cloning plants uses bits of the adult plant as the raw material. Cloning animals currently involves using normal body cells and egg cells as the raw material, though this may change in the future Dolly 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 need for techniques for reproducing high-yielding plants and animals. Also, in developed countries people demand high-quality but inexpensive food, so techniques that reproduce valuable animals and plants more quickly are valued.