6.2.1.1 Mendelian sampling effect on genetic drift

Another reason for allele loss by coincidence has a clear relationship with Mendelian sampling. Even though animals are selected for breeding and mated and they do produce offspring, it is still uncertain which alleles are passed on to the offspring. And in case of multiple offspring: in what proportion they are passed on. Especially in smaller populations allele frequencies are sensitive to this type of genetic drift. Purely by chance allele frequencies change from generation to generation. The smaller the population, the larger the fluctuation in allele frequencies due to genetic drift. Even if all animals participate in breeding, alleles with low frequency run the risk of extinction just because they are not passed on to the offspring by chance. Strangely enough, this can also involve alleles with a positive effect that are under selection! Especially if these alleles have a dominant effect so that heterozygous animals also express the preferred phenotype. At low frequencies most animals that carry the desired allele will be heterozygous. Purely by chance they can all pass on the undesirable allele to their offspring. It sounds strange, but it really happens! Effects of genetic drift can have influences on allele frequencies that are larger than that of selection. Effect of genetic drift is especially important in small populations, where genotypes of individual animals have an influence on the allele frequencies, but it plays a role in populations of all sizes.