Inherited change

Question 1

The fruit fly is a useful organism for studying genetic crosses. Female fruit flies are approximately 2.5 mm long. Males are smaller and possess a distinct black patch on their bodies. Females lay up to 400 eggs which develop into adults in 7 to 14 days. Fruit flies will survive and breed in small flasks containing a simple nutrient medium consisting mainly of sugars.
(a) Use this information to explain two reasons why the fruit fly is a useful organism for studying genetic crosses.

Answer 1

1. Large number of eggs / offspring / flies (therefore) improves reliability / can use statistical tests / are representative / large sample (size) / reduces sampling error;
2. Small size / (breed) in small flasks / simple nutrient medium (therefore) reduces costs / easily kept / stored;
3. Size / markings / phenotypes (therefore) males / females easy to identify
   4) Short generation time / 7 - 14 days / develop quickly / reproduce quickly (therefore) results obtained quickly / saves times / many generations;

Question 2

Male fruit flies have the sex chromosomes XY and the females have XX. In the fruit fly, a gene for eye colour is carried on the X chromosome. The allele for red eyes, R, is dominant to the allele for white eyes, r. The genetic diagram shows a cross between two fruit flies.
(i) Complete the genetic diagram for this cross.

Phenotypes of parents red-eyed female white-eyed male
Genotype of parents ______________ × ______________
Gametes _____and_____ _____and_____
Phenotypes of offspring red-eyed females and red-eyed males
Genotype of offspring ______________ ______________

Answer 2

1.      XRXR and XrY;
All marking points are completely independent. Allow crosses from the following parents for a possible three marks:
XRXR and Xr-
XRXR and XrY;
RR and rY / rY−
RR and r− or RR and r
2.      XR and XR plus X r and Y;
3.      XRXr and XRY;
OR
1.      XRXr and XrY;
OR
XRXr and Xr−
XRXr and XrY;
2.      XR and Xr plus Xr and Y;
Rr and rY / rY−
Rr and r− or Rr and r
Accept different symbols e.g. W and w
2. Accept gametes in a punnet square
3.      XRXr and XRY;

Question 3

The number of red-eyed females and red-eyed males in the offspring was counted. The observed ratio of red-eyed females to red-eyed males was similar to, but not the same as, the expected ratio. Suggest one reason why observed ratios are often not the same as expected ratios.
______________________________________________________________

Answer 3

Fertilisation is random / fusion of gametes is random / small / not large population / sample / selection advantage / disadvantage / lethal alleles

Question 4

Male fruit flies are more likely than female fruit flies to show a phenotype produced by a recessive allele carried on the X chromosome. Explain why.

Answer 4

1. Males have one allele;
2. Females need two recessive alleles / must be homozygous recessive / could have dominant and recessive alleles / could be heterozygous / carriers;

Question 5

Researchers investigated some characteristics of people from different parts of England. In the north of England they selected 200 people and recorded their phenotypes for three different characteristics.
Their results are shown in the figure below.

Phenotype produced
by dominant allele	Number of
people	Phenotype produced
by recessive allele	Number of
people
Tongue roller	131	Non-tongue roller	58
Right-handed	182	Left-handed	14
Straight thumb	142	Hitch-hiker thumb	50
(a)     Calculate the ratio of straight thumb to hitch-hiker thumb in this study.

Answer 5

2.84:1

Question 6

The numbers for the tongue rolling and thumb characteristics do not add up to 200.
For each characteristic suggest one reason why the numbers do not add up to 200.
Tongue rolling

Thumb

Answer 6

1. Some embarrassed / some not willing to show tongue / cannot tell;
2. Could not decide whether thumb was straight or not / thumb bending is judgemental / subjective

Question 7

One student looked at the researchers’ results and concluded that 91% of people in the UK are right-handed.
Do you agree with this conclusion? Give reasons for your answer.

Answer 7

) 1. (No) - should be 92.9% / should be calculated from 182 out of 196 / should not be calculated from 182 out of 200;

2. (Yes) – assumes 4 out of 200 use either hand;
3. (But) sample may not be representative;
4. Small sample size / only sampled 200;

Question 8

Explain what is meant by the term phenotype.

Answer 8

. Expression / appearance / characteristic due to genes and environment

Question 9

Tay-Sachs disease is a human inherited disorder. Sufferers of this disease often die during childhood. The allele for Tay-Sachs disease t, is recessive to allele T, present in unaffected individuals. The diagram shows the inheritance of Tay-Sachs in one family.

Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is recessive.

Answer 9

1. 3 and 4 and 9 / 11 / affected offspring;

2. Both 3 and 4 are carriers / heterozygous;

Question 10

Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is not on the X chromosome.

Answer 10

1. 11 is affected, 3 is not;
   1 Accept: 3 / unaffected father / parents produce
   an affected daughter
2. Accept: 3 and 4 would only produce unaffected females
3. 3 / father of 11 does not have a recessive allele on his X chromosome / Xt;
4. Answers must be in context of alleles

Question 11

In a human population, one in every 1000 children born had Tay-Sachs disease. Use the Hardy-Weinberg equation to calculate the percentage of this population you would expect to be heterozygous for this gene. Show your working.

Answer 11

5.8 − 6.2%

Question 12

The actual percentage of heterozygotes is likely to be lower in future generations than the answer to part (c)(i). Explain why.

Answer 12

Affected individuals (usually) do not reproduce / die during childhood / do not pass on allele / genetic screening

Question 13

Suggest advantages of using island populations in this investigation.

Answer 13

) 1. Can record all individuals on (small) islands;

2. (So) no / less sampling error;
3. (Maybe) different rates of mutation / different selection pressures / different environmental conditions;
4. Inbreeding / breeding with close relatives (more likely);
5. (Little) gene flow / (more chance of) genetic drift

Question 14

The geneticist concluded that arm-folding is not determined by a single gene with a dominant allele and a recessive allele.
Use information from Figure 2 to explain why she reached this conclusion.

Answer 14

1. If R is recessive, R × R parents cannot produce L offspring;
2. If L is recessive, L × L parents cannot produce R offspring;
3. R × R and L × L parents produce both types of offspring;

Question 15

In another study, the geneticist investigated arm-folding in genetically identical twins.
Data from this study supported her conclusion from the island study.
Suggest the evidence she found that supported her conclusion.

Answer 15

Both L and R in a set of twins / (some) twins show different arm-folding

Question 16

In birds, males are XX and females are XY.
(a) Use this information to explain why recessive, sex-linked characteristics are more common in female birds than in male birds.

Answer 16

(Recessive) allele is always expressed in females / females have one
(recessive) allele / males need two recessive alleles / males need to be
homozygous recessive / males could have dominant and recessive alleles /
be heterozygous / carriers

Question 17

Feather colour in one species of chicken is controlled by a pair of codominant alleles which are not sex-linked. The allele CB codes for black feathers and the allele CW codes for white feathers. Heterozygous chickens are blue-feathered.
On a farm, 4% of the chickens were black-feathered. Use the Hardy-Weinberg equation to calculate the percentage of this population that you would expect to be blue-feathered. Show your working.

Answer 17

Correct answer of 32 (%)

Question 18

Explain why primer A3 and primer A4 only bind to specific DNA fragments.

Answer 18

1. Each has / they have a specific base sequence;

2. That is complementary (to allele r or R).

Question 19

The scientists wanted to know on which chromosome the gene with alleles R and r was located. From the flies with genotype RR, they obtained cells that were in mitosis and added a labelled DNA probe specific for allele R. They then looked at the cells under an optical microscope.
Explain why they used cells that were in mitosis.

Answer 19

(Cells in mitosis) chromosomes visible;

(So) can see which chromosome DNA probe attached to

Question 20

Explain why the scientists carried out the control experiment with the non-resistant flies.

Answer 20

1) For comparison with resistant flies / other (two) experiments
/ groups;

2. To see death rate (in non-resistant) / to see effect of insecticide in non-resistant / normal flies.

Question 21

The allele for tongue-rolling, T, is dominant to the allele for non-tongue rolling, t.
The gene controlling tongue-rolling is not sex-linked. Individuals 10 and 11 are both heterozygous for tongue-rolling.
What is the probability that individuals 10 and 11 will produce a male child who is colour blind and a non-tongue roller?

Answer 21

0.0625 / 6.25% / 1⁄16

Question 22

In a population, the frequency of the allele for tongue-rolling, T, is 0.4.
Use the Hardy-Weinberg equation to calculate the percentage of people in this population that are heterozygous for tongue-rolling.

Answer 22

48(%)

Question 23

In cats, males are XY and females are XX. A gene on the X chromosome controls fur colour in cats. The allele G codes for ginger fur and the allele B codes for black fur. These alleles are codominant. Heterozygous females have ginger and black patches of fur and their phenotype is described as tortoiseshell.
(a) Explain what is meant by codominant alleles.

b)) Male cats with a tortoiseshell phenotype do not usually occur. Explain why.

Answer 23

(a) Both alleles are expressed / shown (in the phenotype).

1
(b) Only possess one allele / Y chromosome does not carry allele / gene / can’t be heterozygous.

Question 24

Polydactyly in cats is an inherited condition in which cats have extra toes. The allele for polydactyly is dominant.
(i) In a population, 19% of cats had extra toes. Use the Hardy-Weinberg equation to calculate the frequency of the recessive allele for this gene in this population.
Show your working.

Answer 24

0.9

Question 25

Some cat breeders select for polydactyly. Describe how this would affect the frequencies of the homozygous genotypes for this gene in their breeding populations over time.

Answer 25

Homozygous dominant increases and homozygous recessive decreases.

Question 26

In fruit flies, the genes for body colour and wing length are linked. Explain what this means.

Answer 26

(Genes / loci) on same chromosome