Explain, using a named example, how polygenic inheritance gives rise to continuous variation.

Describe the inheritance of colour blindness in humans.

human skin colour can vary from pale to very dark / amount of melanin varies;
skin colour/melanin controlled by (alleles from) at least three/several genes;
no alleles are dominant / alleles are co-dominant / incomplete dominance;
many different possible combinations of alleles;
skin colour controlled by cumulative effect/combination of genes/alleles;
Award the above marking points for any other valid example.

sex linked condition;
carried on an X chromosome / absent from Y chromosome;
if present in male causes colour blindness;
(allele is) recessive so heterozygous females are not colour blind;
homozygous females are colour blind;
Do not allow carried on sex chromosome.

Answers were varied in quality. Some candidates were not clear about the nature of continuous variation and therefore either described how a small number of skin colours could arise, or described another example of variation with only a small number of phenotypic variants. The best answers explained how continuous variation results from the alleles of different genes acting in combination, with no single allele being dominant over the others. As there is considerable uncertainty about the number of genes influencing the quantity of melanin in human skin, the mark scheme accepted a wide range of answers.

This was well known by the stronger candidates, who had no difficulty in scoring three marks. There were some long answers describing particular mating and the offspring that they could produce, which sometimes scored few marks, as they did not make general points about the inheritance of colour blindness. Where crosses are used in an answer to a general question about the inheritance of a trait, they should be used to exemplify the pattern of inheritance, with annotation to make general points, rather than focusing too much on specific ratios.