A2 2023 — CCEA A2 Biology Practice Questions

Question 1

A cross between two heterozygous black, short-haired guinea pigs produced 96 offspring. Predict the phenotypic ratio and explain how you would test whether the observed results differ significantly from your prediction.

Accepted Answer

Both parents are BbSs, producing gametes BS, Bs, bS and bs. A 4 × 4 Punnett square gives the expected ratio 9 black short : 3 black long : 3 brown short : 1 brown long, which for 96 offspring is 54 : 18 : 18 : 6. To test the observed results I would carry out a chi-squared test, χ² = Σ (O − E)² / E, using these expected values. There are four phenotype classes, so degrees of freedom = 3 and the critical value at p = 0.05 is 7.82. If the calculated χ² is less than 7.82 the difference between observed and expected is not significant and is due to chance, so the null hypothesis is accepted.

Question 2

Explain how reproductive isolation can lead to the formation of a new species.

Accepted Answer

A barrier — for example a geographical barrier such as a river, or a behavioural difference in courtship — prevents the two populations from interbreeding, so there is no gene flow and no exchange of alleles between them. Different mutations arise in each population and different selection pressures act on them, so different alleles give greater reproductive success in each environment. The frequencies of alleles therefore change in different directions in the two populations over many generations. Eventually the populations are so genetically and phenotypically different that even if brought back together they could no longer interbreed to produce fertile offspring, so they are separate species.

Question 3

A population of snails shows two shell colour phenotypes. Explain how natural selection could increase the frequency of the darker phenotype, and explain how you would test whether the two phenotypes occur in the ratio predicted by a genetic cross.

Accepted Answer

Shell colour varies in the population because different alleles are present. If the environment changes so that the background is darker, the darker snails are better camouflaged and less likely to be seen by predators, so they are more likely to survive and reproduce. They pass the allele for dark shell to their offspring, so its frequency increases over successive generations - directional selection. To test whether the two phenotypes occur in the predicted ratio I would use the chi-squared test, calculating the sum of (observed - expected) squared divided by expected. With two phenotypes there is one degree of freedom, so I would compare the calculated value with the critical value at the 0.05 probability level. If the calculated value is less than the critical value, the difference between the observed and expected numbers is not significant and is due to chance.

A2 2023: CCEA Biology practice questions

Sit the questions

A cross between two heterozygous black, short-haired guinea pigs produced 96 offspring. Predict the phenotypic ratio and explain how you would test whether the observed results differ significantly from your prediction.

Read the full walkthrough

More practice sets