Question

What are the types of genetic load in populations?

Answer 1

Before learning the types of genetic load, it is important for us to grab the concept of the genetic load itself. In simple words, the genetic load may be defined as the presence of unfavourable genes in the overall genome of a population. The term ‘load’ is thus used in this case, which signifies the fact that the population carries these genes as a “load” which are of no use for adaptational benefits of the population to the environment but only has negative impacts.

Complete answer:
In genetic load, the average fitness of a gene (i.e., to determine that whether the gene is a load for the population or it is useful for the population) is calculated with respect to the fitness of a reference gene or genotype, that is either the best in the population, or hypothetically assumed to be the best based on situations. Thus, organisms/populations that will have less genetic load will have more offspring than the ones with a greater genetic load.

Now, genetic load in a population is of three main types:
i) Mutational Genetic Load: This load arises due to mutation in the genes that give a negative impact of the adaptorial effects of the population.
ii) Balanced Genetic Load: This type of load arises when the selection in the population occurs in favour of heterozygous or homozygous. This means that the load is neither negative nor too positive for the population but overall, the population can survive well along with this load. The load is balanced between its negative and positive impacts. The selection of heterozygous individuals for sickle cell anaemia is an example of this type.
iii) Substitutional Genetic Load: This type of load arises due to sudden changes in environmental conditions. The alleles or genes that were favored in the previous conditions now become the load for that population. A well-known example for this type is industrial melanism.

Note: One must be well acquainted with the causes of genetic load occurrence in a population. The causes are:
1. Deleterious mutations
2. Beneficial mutations
3. Inbreeding acts
4.Recombination/segregation
5. Migration

Of these, everything else is to some extent understandable, except point no. 3. Inbreeding is defined as mating between organisms that have a common ancestor for 4-5 generations. This leads to a phenomenon known as inbreeding depression, where the population loses its fertility, and adaptability.