17.1.3—Genetic basis of variation

Genetic variation is variation in the DNA base sequence or allele combinations between individuals of a species. It can create different inherited possibilities for phenotype; environmental conditions then affect how those possibilities are expressed.

• Mutation changes a DNA base sequence and can create a new allele. For the change to be inherited, it must occur in genetic material that contributes to gametes.
• Crossing over, independent assortment and random fertilisation create new combinations of existing alleles, rather than automatically creating new alleles.
• A gene is a DNA sequence at a locus; different alleles are alternative versions of that sequence. Their effects can be large at one locus or small and additive across several genes.

Discontinuous phenotypes are commonly associated with a large effect at one gene locus, whereas continuous phenotypes commonly reflect many small genetic effects together with environmental influence. This is a tendency within the syllabus boundary, not a cause that can be diagnosed from the phenotype alone.

Recombination reshuffles existing alleles; mutation can introduce a new allele. Environmental effects can change phenotype without changing the inherited DNA and are not themselves inherited as the same phenotype. Detailed t-test calculations and natural-selection frequency changes are outside this card.