Explain patterns of human variation, including multiple alleles, polygenic inheritance and sex linkage, and distinguish continuous from discontinuous variation

What this dot point is asking

WACE wants you to recognise and explain the patterns behind human variation, building on the gene-and-allele vocabulary. The key division is between traits that fall into categories and traits that vary smoothly, and the genetics that produce each.

Multiple alleles: the ABO blood groups

Some genes have more than two alleles in the population, called multiple alleles, although any one person still carries only two. The classic example is the ABO blood group gene, which has three alleles: IA, IB and i. Alleles IA and IB are codominant (both expressed when present together) and both are dominant over i. This produces four blood group phenotypes (A, B, AB and O) from six genotypes. This is discontinuous variation: you are one group or another, with no in-between.

Polygenic inheritance and continuous variation

Many human traits are controlled by several genes acting together, called polygenic inheritance. Each gene adds a small effect, so the trait varies across a smooth range rather than in categories. Height, body mass and skin colour are polygenic. Because so many alleles combine, and because the environment also affects these traits, the population shows continuous variation with a normal (bell-shaped) distribution: most people are near the middle and fewer at the extremes.

Sex-linked inheritance

Genes carried on the sex chromosomes show sex linkage. Most sex-linked genes are on the X chromosome (X-linked). Because females are XX and males are XY, the inheritance pattern differs between the sexes. A male has only one X chromosome, so a single recessive allele on it is expressed (he has no second X to mask it). A female needs two copies of a recessive allele to show the trait, because a normal allele on her other X can mask it. This is why X-linked recessive conditions such as red-green colour blindness and haemophilia are more common in males. Females with one copy are carriers.

Environmental influence

Variation is not purely genetic. The environment affects how genes are expressed: diet affects height and mass, ultraviolet exposure affects skin colour, and exercise affects muscle. Continuous (polygenic) traits are the most affected by the environment, which adds to the smooth spread of values. Discontinuous traits such as blood group are essentially unaffected by the environment.

How this maps to the exam

Expect questions that ask you to classify a trait as continuous or discontinuous and justify it, work with ABO blood group genotypes and phenotypes, or explain why a sex-linked condition is more common in one sex. You may be given a population distribution and asked whether it shows continuous or discontinuous variation and what that implies about the genetics.