Discontinuous and continuous variation

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Discontinuous variation

Describes the qualitative differences between phenotypes. There are no intermediate categories eg blood group, male or female.

Different alleles at a single gene locus have large effects on the phenotype

Different gene loci have quite different effects on the phenotype

Eg= codominance, dominance and recessive patterns of inheritance.

Continuous variation

This describes quantitative differences between phenotypes. These are phenotypic differences where there is a wide range of variation within the population, with no distinct categories. Eg- height, mass, length, yield and size.

Traits exhibiting continuous variation are controlled by two or more genes

Each gene provides an additive component to the phenotype

Different alleles at each gene locus have a small effect on the phenotype

A large number of different genes may have a combined effect on the phenotype.

These are known as polygenes and the characteristics they control is described as polygenic. These genes are unlinked they are on separate chromosomes.