Patterns of inheritance and variation: WACE Year 12 Human Biology Unit 4

Syllabus dot point

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

8 min readUpdated 2 Jun 2026

Quick answer

Human traits follow different inheritance patterns. Multiple alleles (more than two versions of a gene, as in ABO blood groups) and single-gene traits give discontinuous variation, with distinct categories. Polygenic traits, controlled by many genes plus the environment (such as height and skin colour), give continuous variation across a smooth range. Sex-linked traits, carried on the X chromosome, affect males and females differently.

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.

Multiple alleles

Multiple alleles exist when a gene has more than two versions present in a population, even though each diploid individual carries only two of them. The ABO blood group gene, with its IA, IB and i alleles, is the standard human example and produces discontinuous variation in four distinct blood group categories.

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.

Continuous variation is polygenic and environment-sensitive

Continuous variation (a smooth range, like height) is produced by polygenic inheritance and is strongly influenced by the environment. Discontinuous variation (distinct categories, like blood group) is usually controlled by one gene and is little affected by the environment. Linking the pattern of variation to its genetic cause is what a marker is looking for.

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.

Explaining why colour blindness is more common in males

Tracing an X-linked recessive trait

Red-green colour blindness is caused by a recessive allele on the X chromosome. A male is XY, so he has only one X chromosome; if it carries the recessive allele, he is colour blind because there is no second X to carry a normal allele. A female is XX, so she has two X chromosomes; she is only colour blind if both carry the recessive allele, which is less likely. A female with one recessive and one normal allele is a carrier with normal vision. A marker wants you to explain the pattern using the single X in males versus the two X chromosomes in females, not just to state that it is more common in males.

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.

Saying continuous variation is controlled by a single gene

A frequent error is mismatching the pattern and the genetics. Continuous variation (a smooth range) is polygenic and environment-influenced; discontinuous variation (distinct categories) is usually single-gene. Another common slip is claiming a male can be a carrier of an X-linked recessive condition; because he has only one X, he either has the allele and shows the trait or does not have it at all. Only females (XX) can be carriers.

Exam-style questions

Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

WACE 2022-style6 marks

Red-green colour blindness is a sex-linked recessive condition carried on the X chromosome. A woman who is a carrier has children with a man who has normal colour vision. Using a Punnett square, determine the expected proportions of children who are colour blind, and explain why this condition is more common in males.

Show worked answer →

A 6 mark response needs the correct cross, the offspring ratios, and the reason for the sex difference.

Genotypes: Let $X^N$ be the normal allele and $X^n$ the colour-blind allele. Carrier mother is $X^N X^n$ ; normal father is $X^N Y$ .
Punnett square: Crossing $X^N X^n$ with $X^N Y$ gives offspring: $X^N X^N$ , $X^N X^n$ , $X^N Y$ , $X^n Y$ .
Proportions: Daughters: half are $X^N X^N$ (normal) and half are $X^N X^n$ (carriers); no daughters are colour blind. Sons: half are $X^N Y$ (normal) and half are $X^n Y$ (colour blind). So overall 1 in 4 of all children (half the sons) are colour blind.
Why more common in males: Males have only one X chromosome, so a single recessive allele is expressed. Females need two copies of the recessive allele to be affected, which is much less likely.

Markers reward the correct genotypes, a clear Punnett square, the ratios, and the single-X explanation.

WACE 2024-style4 marks

Distinguish between continuous and discontinuous variation, giving one human example of each, and explain why polygenic traits tend to show continuous variation.

Show worked answer →

A 4 mark answer needs the distinction, examples, and the polygenic link.

Continuous variation: A trait that varies smoothly across a range with many intermediate values, for example human height or skin colour.
Discontinuous variation: A trait that falls into distinct categories with no intermediates, for example ABO blood group.
Why polygenic traits are continuous: Polygenic traits are controlled by many genes, often with environmental influence. The combined effect of many genes produces a wide range of small differences, giving a smooth, continuous range rather than a few clear categories.

Markers reward the smooth-range versus distinct-categories distinction, correct examples, and the many-genes explanation.

Practise this

Sources & how we know this

Human Biology ATAR course: syllabus and support materials — SCSA
Past ATAR course examinations — SCSA