genetic load

The proportion of disadvantageous genes that are present in and sustained by a population. Genetic load has three components; mutational load, segregational load, and substitutional load. The first arises because most mutations are harmful. The second may occur in cases of heterozygous advantage, since selling a fit heterozygote Aa generates deleterious AA and aa offspring. The third, also called environmental load or frequency-dependent load, occurs during transient *polymorphism.