Edexcel A-Level Biology: Origins of Genetic Variation

Meiosis is the type of nuclear division that produces gametes, and it is a major source of genetic variation within a population.

Describe and explain how meiosis produces genetically different gametes from a single diploid parent cell. In your answer you should refer to crossing over, independent assortment and the effect each has on the genetic make-up of the gametes.

(6 marks)

In a species of ornamental plant, two unlinked genes control seed appearance: one gene controls colour (yellow is dominant to green) and the other controls shape (round is dominant to wrinkled). A breeder crossed two plants that were heterozygous at both loci. From this cross, the breeder predicted the offspring would appear in a 9 : 3 : 3 : 1 ratio of the four phenotypes.

A total of 640 offspring were scored. The observed numbers of each phenotype are shown below.

Phenotype	Observed number (O)
Yellow, round	342
Yellow, wrinkled	118
Green, round	132
Green, wrinkled	48

The breeder used the chi-squared test, $\chi^2 = \sum \frac{(O - E)^2}{E}$χ2=∑E(O−E)2​ to decide whether the observed results fit the expected 9 : 3 : 3 : 1 ratio.

(a) Calculate the expected number (E) of each phenotype from the 9 : 3 : 3 : 1 ratio, then calculate the value of $\chi^2$χ2. Show your working. (4 marks)

(b) The critical value of $\chi^2$χ2 at the 0.05 probability level for this test is 7.81 (degrees of freedom = 3). Using your value of $\chi^2$χ2, state whether the null hypothesis should be accepted or rejected, and explain what this tells the breeder about the inheritance of these two genes. (2 marks)

A recessive allele causes a metabolic condition in a population of farmed fish. Affected fish are homozygous recessive. In a large breeding population, the recording team scored every fish and found the data below.

Phenotype	Number of fish
Unaffected (dominant phenotype)	9 216
Affected (recessive phenotype)	784
Total	10 000

You may assume the population is in Hardy-Weinberg equilibrium. The Hardy-Weinberg equations are $p + q = 1$p+q=1 and $p^2 + 2pq + q^2 = 1$p2+2pq+q2=1, where $q$q is the frequency of the recessive allele.

(a) Calculate the frequency of the recessive allele, $q$q, and the frequency of the dominant allele, $p$p. Show your working. (2 marks)

(b) Calculate the expected number of heterozygous (carrier) fish in this population of 10 000. Show your working. (3 marks)

A small population of lizards lives on a remote island. A storm washes a single floating log carrying just six of these lizards onto a previously uninhabited neighbouring island, where they establish a new breeding population. Over the following decades, biologists notice that the new island population has a much higher frequency of a rare blue-throat allele than the large original population, even though the blue throat gives no survival or mating advantage on the new island.

Suggest and explain why the frequency of the blue-throat allele became so much higher in the new population than in the original one. (5 marks)

In a species of insect, the sex-determination system is the same as in humans: females are XX and males are XY. A recessive allele on the X chromosome causes a pale-eye condition. A breeder notices that, across many broods, pale eyes appear far more often in male insects than in female insects.

Explain why the pale-eye condition is much more common in males than in females in this species. (4 marks)

Gene mutations are changes to the base sequence of DNA. A substitution mutation replaces one base with another, while a deletion mutation removes a base from the sequence.

Describe why a single base deletion is often more harmful to the resulting protein than a single base substitution. (3 marks)