Origins of genetic variationEdexcel A-Level Biology B: Topic test

20 questions, 54 marks

Edexcel A-Level Biology B

Origins of genetic variation topic test

Total 54 marks

Name

Class

Date

1
Meiosis in cells of a human testis produces sperm cells. During prophase I the homologous chromosomes pair up and exchange sections of DNA.
(a)
Between which chromosomes does crossing over occur?
[1 mark]
ANon-sister chromatids of a homologous pair
BSister chromatids of the same chromosome
CChromosomes from different homologous pairs
DThe two daughter cells formed at the end of meiosis I
(b)
Which process during meiosis also produces new combinations of alleles in the sperm cells?
[1 mark]
AReplication of the DNA before meiosis
BRandom assortment of the homologous chromosomes at metaphase I
CFusion of a sperm cell with an egg cell
DMutation of the DNA during interphase
(c)
Explain how crossing over increases the genetic variation of the sperm cells.
[2 marks]
Total for question 1: 4 marks
2
In shorthorn cattle, coat colour is controlled by one gene with two codominant alleles, $C^{R}$ for red and $C^{W}$ for white. Heterozygous cattle are roan, with a mixture of red and white hairs.
(a)
A red bull is crossed with a white cow. What are the expected phenotypes of the calves?
[1 mark]
AAll red
BAll white
CAll roan
DHalf red and half white
(b)
Two roan cattle are crossed. What are the expected phenotype ratios in the calves?
[1 mark]
A3 red : 1 white
B1 red : 1 white
C1 roan : 1 white
D1 red : 2 roan : 1 white
(c)
Explain, using the term codominance, why the calves from the cross between the red bull and the white cow are roan.
[2 marks]
Total for question 2: 4 marks
3
In sweet peas, purple flowers (P) are dominant to red flowers (p) and long pollen grains (L) are dominant to round pollen grains (l). A plant of genotype PpLl, produced by crossing PPLL with ppll, was test crossed with a plant of genotype ppll. The 400 offspring were 172 purple long, 25 purple round, 22 red long and 181 red round.
(a)
Identify the recombinant phenotypes and calculate the percentage of the offspring that are recombinants.
[3 marks]
(b)
The two genes assort independently only if they are on different chromosomes. Explain why the offspring are not in the ratio 1 : 1 : 1 : 1, and why recombinants are produced at all.
[4 marks]
Total for question 3: 7 marks
4
In a region of West Africa where malaria is common, about 1 in 25 babies are born with sickle cell anaemia (HbS HbS), and many more are carriers (HbA HbS). Carriers are less likely to die of severe malaria than people with genotype HbA HbA, whereas many people with HbS HbS die young. Twenty people from the region, including several carriers, later settle on a remote island where malaria does not occur.
(a)
Explain how selection pressures keep the HbS allele at a high frequency in the West African region.
[6 marks]
(b)
Explain how the frequency of the HbS allele in the island population may come to differ from that in the original region, and why it may change over many generations.
[6 marks]
Total for question 4: 12 marks
5
A woman's skin cells accumulate mutations because of exposure to ultraviolet light. A different mutation arises in a cell of one of her ovaries, and that cell then divides by meiosis to form an egg cell.
(a)
Which mutation could be passed on to the woman's children?
[1 mark]
AA mutation in a skin cell that has divided many times
BA mutation in the cell that divides by meiosis to form an egg cell
CA mutation in a white blood cell
DA mutation in a liver cell
(b)
What is the origin of a new allele in the egg cell?
[1 mark]
ACrossing over between chromatids
BRandom assortment of chromosomes
CMutation of the DNA
DRandom fertilisation by a sperm
(c)
Explain why mutation is the source of new alleles, whereas meiosis and random fertilisation only produce new combinations of alleles.
[2 marks]
Total for question 5: 4 marks
6
Red-green colour blindness is caused by a recessive allele, $X^{b}$ , on the X chromosome. The dominant allele, $X^{B}$ , gives normal colour vision. A woman who is a carrier ( $X^{B}X^{b}$ ) and a man with normal colour vision ( $X^{B}Y$ ) have children.
(a)
What is the probability that a child of these parents, of either sex, is colour blind?
[1 mark]
A $\frac{1}{2}$
B $\frac{1}{8}$
C $\frac{3}{4}$
D $\frac{1}{4}$
(b)
What is the probability that a daughter of these parents is colour blind?
[1 mark]
A0
B $\frac{1}{4}$
C $\frac{1}{2}$
D1
(c)
Explain why red-green colour blindness is more common in men than in women.
[2 marks]
Total for question 6: 4 marks
7
In guinea pigs, black coat (B) is dominant to brown (b) and short hair (S) is dominant to long hair (s). The two genes are on different chromosomes. A breeder crosses two guinea pigs that are both heterozygous for both genes ( $BbSs \times BbSs$ ) and obtains 160 offspring: 84 black short, 36 black long, 28 brown short and 12 brown long.
(a)
Calculate the value of $\chi^{2}$ for these results, using the expected ratio of 9 : 3 : 3 : 1.
[3 marks]
(b)
The critical value of $\chi^{2}$ at the 5% significance level for these data is 7.82. Use your value of $\chi^{2}$ to draw a conclusion about the results.
[4 marks]
Total for question 7: 7 marks
8
On a Pacific island, lizards vary in colour from pale to dark. Colour is controlled by one gene with two alleles: D (dark) and d (pale). Dd lizards are intermediate in colour. The island has areas of pale sand and areas of dark lava rock. Birds that hunt by sight eat lizards that do not match their background, and intermediate lizards are rare.
(a)
Explain how predation by the birds could lead to the lizards forming two distinct colour groups, and how this could eventually lead to the formation of new species.
[6 marks]
(b)
In one year the frequency of the allele $d$ was measured as 0.45, having been 0.30 ten years earlier. Calculate, using the Hardy-Weinberg equation, the expected percentage of heterozygous (intermediate) lizards and of pale lizards in the later year, and explain what the change in allele frequency shows.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).