3.21—Multiple alleles, polygenic inheritance and continuous variation

Some phenotypes depend on several genes or several alleles as well as the environment. Small additive effects from many loci produce a range of values rather than a few discrete categories.

A single-gene trait such as ABO blood group gives discontinuous categories. A polygenic trait such as height or mass is usually continuously distributed because different allele combinations add together and environmental conditions shift the result.

If two genes each contribute a small height effect, an individual carrying more contributing alleles can be taller than one carrying fewer; nutrition and water availability can still move either genotype away from its genetic potential.

Do not assume every gene contributes equally or that a continuous phenotype is purely genetic. “Polygenic” describes many genetic contributors; it does not remove environmental influence.