(d) the genetic basis of continuous and discontinuous variation

The idea

Variation between individuals is either continuous (a smooth range of phenotypes, such as human height or mass) or discontinuous (distinct categories, such as ABO blood group or flower colour). The genetic basis differs. Continuous variation is usually polygenic: many genes, each with a small additive effect, plus environmental influence, give a normal distribution. Discontinuous variation is usually controlled by one gene or a few genes with distinct alleles, so phenotypes fall into clear classes; the environment has little effect on the category. Meiosis also contributes: independent assortment of chromosomes and crossing over create new allele combinations, and random fertilisation adds further genetic variation. Mutation supplies new alleles.

What earns the marks

• Continuous variation is controlled by many genes (polygenic) whose alleles have small additive effects, producing a range of phenotypes.
• Continuous variation is also influenced by the environment, so the phenotype forms a normal distribution rather than discrete classes.
• Discontinuous variation is controlled by one gene or a few genes with distinct alleles, giving clearly separate phenotypic categories.
• Discontinuous variation is little affected by the environment, so individuals fall into distinct groups such as blood group.
• Meiosis increases genetic variation through independent assortment of chromosomes and crossing over between homologous chromosomes.
• Random fertilisation of gametes and mutation as a source of new alleles also contribute to genetic variation.

Common mistakes and how to put them right

• Assuming all continuous variation is purely genetic.

  Put it right: Environment also contributes, which is why the distribution is smooth.

• Thinking discontinuous variation is caused by many genes.

  Put it right: It is typically single-gene or few-gene with distinct alleles.

• Confusing the source of new alleles with reshuffling.

  Put it right: Mutation creates new alleles, whereas meiosis and random fertilisation recombine existing alleles.

Exam tips

• Identify the number of genes involved first, then decide whether environment smooths the phenotype.
• Link each type of variation to a concrete example, such as height for continuous and blood group for discontinuous.
• When asked about genetic variation, name meiosis processes (crossing over, independent assortment) and random fertilisation explicitly.