18.1 Variation

Variation as differences between individuals of the same species. Continuous variation (range — body length, body mass) caused by genes + environment vs discontinuous variation (distinct categories — ABO blood groups, seed shape, seed colour) caused by genes only. Mutation as a genetic change — the source of new alleles. Ionising radiation and certain chemicals increase the mutation rate. HL extension: gene mutation defined as a random change in the DNA base sequence; four sources of genetic variation in populations (mutation, meiosis, random mating, random fertilisation).