What is Inbreeding Depression?
Inbreeding depression is the reduction within the biological fitness of the individuals of a population that is a result of inbreeding. Inbreeding depression refers to the decrease or loss of fitness and strength which is specially caused because of the inbreeding. In simpler form, the mating between the relatives in a small population is common and this may lower the population’s ability to persist and reproduce which is referred to as inbreeding depression.
This phenomenon occurs in all the wild animals, plants and also in humans, representing that genetic differences in fitness traits exist both within and among the normal populations. Inbreeding depression plays a vital role in crop breeding and within the evolution of outcrossing mating systems.
The biological fitness is an organism’s ability to survive and conserve its genetic material. It is the result of population bottleneck. The higher the genetic variations in a breeding population, the fewer are the chances for it to suffer from inbreeding depression.
Inbreeding depression varies across mating systems. A lower degree of inbreeding depression is exhibited in hermaphrodite species C.elegans. The outcrossing nematode such as C.remanei suffers severely from inbreeding depression.
The term homozygous refers to an offspring having two of the same allele either dominant or recessive. In other words, the term homozygous refers to a particular gene that has identical alleles on both homologous chromosomes.
Inbreeding Depression in Plants
Onion, carrot, maize, sunflower, etc. are few examples of plants produced by inbreeding depression either by the self-pollination or cross-pollination process. This phenomenon is observed in several other plant species that are further grouped based on the following four categories.
High Inbreeding Depression
A large proportion of plants produced by self-pollination cause a severe inbreeding depression and exhibit a lethal effect. It is very hard to maintain the breeding line after three to four generations due to the loss of vigour and fertility. These are mainly seen in Alfalfa of the pea family and carrots.
Moderate Inbreeding Depression
Along with the lethal effects, sublethal effects are seen in the offspring produced by self-pollination. There is a considerable decrease in fertility, as several lines produced are very poor and lost. Maize, pearl, millet are some of the examples of plants showing moderate inbreeding depression.
Low Inbreeding Depression
A minor proportion of plants exhibit lethal characteristics. The loss of vigour and fertility is lesser. Onion, squash, pumpkin, sunflower are some of the examples of plants showing low inbreeding depression.
No Inbreeding Depression
This phenomenon is mainly seen in the self- pollinated species as they do not show any inbreeding depression even though they do not show heterosis. It is because they reproduce both by self-pollination with developed homozygous balance and cross-pollination with heterozygous balance.
Inbreeding Depression in Vipera Berus (Animal Breeding)
Vipera berus is usually referred to as European adder or European viper. It is a venomous snake that is extremely widespread in Western Europe and East Asia.
When a group of 40 Vipera berus experienced inbreeding depression, a greater number of deformed and stillborn offspring were produced in the isolated population than in the larger population. Once after introducing the animal breeding from other inhabitants into the isolated population, they reproduced by recovering a developed portion of viable offspring.
The reason behind the recovery is that the species of Vipera berus with a single recessive deleterious or detrimental allele will be healthier and can reduce the carrier’s fitness. Therefore, fewer copies wind up in the next generation.
Finally, inbreeding depression is specially seen only in smaller populations instead of the larger populations. Because in the smaller population, when the individuals mate, there are possibilities that an offspring inheriting two copies of the same recessive deleterious allele will suffer the significances of expressing the deleterious allele.
In humans, this phenomenon is very rare. It is especially found in the case where the marriages between closely related ancestries are performed.