3.19 - 3.20 - Variation

3.19 — Phenotype and many genes

Variation means differences between individuals of the same species. Some differences can be observed directly, such as flower colour, while others can be measured, such as height.

A simple genetic cross follows different alleles of one gene. That model is useful for some characteristics, but most phenotypic features are influenced by multiple genes rather than by single-gene inheritance. A feature influenced by multiple genes is described as polygenic.

Height is a helpful example. An individual inherits a combination of alleles at many genes that can influence height. Nutrition and health during growth can also influence the final phenotype, so two people with different allele combinations may differ in height, and people with similar genetic influences may still grow to different heights in different environments.

This does not mean that genes and environment contribute equally to every feature. It means that a phenotype should not automatically be explained by one gene or by genes alone.

3.20 — Genetic and environmental variation

Variation can be grouped by what causes the difference in phenotype.

Genetic variation is caused by differences in genetic information. It has two important sources in this lesson:

1. A mutation is a change in DNA. A mutation can create a new genetic variant, including a new allele.
2. Sexual reproduction gives offspring combinations of alleles inherited from two parents. Gametes carry different allele combinations, and fertilisation combines one gamete from each parent, so offspring are genetically different from their parents and usually from their siblings.