Genetic load is a measure of the number of disadvantageous genes in a population or of the damage done to the population by certain genetic factors. Disadvantageous genes reduce fitness by mortality, morbidity or sterility in heterozygotes, if the gene is dominant and in homozygotes, if it is recessive. Thus, it is related to Darwinian fitness.
The genetic load can have two sources, one, mutational and the other, segregational and recombination. The mutational load is due to recurrent mutations. Most of the mutations are disadvantageous. It may be lethal also. While dealing with mutational load several factors like the number of deleterious genes, the nature of mutant alleles (recessive alleles, dominant alleles, partially lethal alleles etc.) and the occurrence of mutations are to be taken into consideration. The frequencies of mutant genes are, however, low. But there are possibilities of an increase of this type of load due to reduction of selection with the progress of civilization.
Segregation leads to the formation of different genotypes, of which some are less fit. There may be a heterozygous advantage also as in balanced polymorphism. Blood group incompatibility is another component of genetic load. One should note the differential fitness of some genotypes under different environmental conditions. A comparative study of the offspring of consanguineous and non-consanguineous marriages can provide useful information on genetic load due to disadvantageous genes. The number and proportion of homozygous offspring are relatively high as a result of the first type of mating. We have already noted that the rate of mortality and that of inherited diseases are relatively high in the first category.
The genetic load of disadvantageous recessive genes is usually expressed by lethal equivalents. A lethal equivalent is one deleterious gene that causes death in a homozygous state. Two genes, when each of them in a homozygous combination will lead to death in 50% of the cases. It is said that the total genetic damage is 1.5 to 2.5 lethal equivalents per gamete and per zygote is 3 to 5.