When organisms reproduce, they pass on their alleles to the next generation. There are many factors that can affect how allele frequencies change over time in a population.

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

1. [6] any six from: for either 1 random / chance (event) ; 2 not (natural) selection / not caused by selection pressure ; 3 loss of (some) alleles ; I loss of gene 4 (so) increase in frequency of, one / small number of, allele(s) or increase in homozygosity / decrease in heterozygosity; 5 small(er) gene pool / reduction in genetic variation; genetic drift 6 (random), fusion of gametes / mating / death of individual/mutation; 7 larger effect in small population ; founder effect 8 small number of individuals, isolated / start new population / migrate ; 9 not all alleles present from original population / alleles not representative of original population ; 10 AVP; e.g. ref. to bottleneck effect genetic drift occurs gradually over time founder effect is a one-off event