Genetic Drift and Genotype Frequencies

The question asks about a factor that leads to a decrease in the frequency of heterozygous genotypes and a simultaneous increase in the frequency of homozygous genotypes, specifically under conditions of random mating.

Let's examine the options provided:

• Genetic drift: This refers to random fluctuations in allele frequencies from one generation to the next, particularly significant in small populations. Genetic drift can lead to the loss of some alleles and the fixation (frequency reaching 100%) of others over time. When an allele becomes fixed, the population becomes entirely homozygous for that allele at that locus. If an allele is lost, the corresponding heterozygote and homozygote genotypes disappear. Even under random mating, these random shifts in allele frequencies caused by drift directly impact genotype frequencies according to the Hardy-Weinberg principle ($p^2 + 2pq + q^2 = 1$), leading to a decrease in heterozygosity ($2pq$) and an increase in homozygosity ($p^2$ and $q^2$) as allele frequencies shift towards 0 or 1.
• Intense inbreeding: Inbreeding is mating between individuals who are more closely related than average in the population. This is a form of non-random mating. Inbreeding does significantly increase the frequency of homozygous genotypes and decrease the frequency of heterozygous genotypes. However, the question specifically mentions "random mating," which is the opposite of inbreeding. Therefore, inbreeding cannot be the correct answer in the context of random mating.
• Reverse mutation: Mutation is the ultimate source of genetic variation, but reverse mutation (changing an allele back to a previous form) is a specific type of mutation. While mutation affects allele frequencies over long periods, it doesn't inherently cause a general shift towards increased homozygosity and decreased heterozygosity under random mating.
• Founder effect: The founder effect is a special case of genetic drift that occurs when a new population is established by a small number of individuals. The allele frequencies in the new population may be very different from the source population due to the small sample size. This effect is a result of genetic drift, not the general mechanism causing the frequency shift under random mating across any population (small or large, though more pronounced in small ones).

Considering the context of random mating, genetic drift is the evolutionary force that, due to random sampling of alleles in finite populations, leads to changes in allele frequencies. These changes often result in the loss of rare alleles and the fixation of common ones, ultimately causing a decrease in heterozygosity and an increase in homozygosity over generations, even if mating is random within that population.

Therefore, the phenomenon that causes a reduction in the frequency of heterozygous genotype with a concomitant increase in the frequency of homozygous genotype in the context of random mating is genetic drift.