Inbreeding is the process of producing their offspring through sexual reproduction either through the breeding or mating system of individuals which are closely related genetically. This process results in homozygous.
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 refers to decrease or loss of vigor and fertility as a result of inbreeding. The main effect of inbreeding depression results in the increase in the homozygous in the progeny, which is proportionate to the degree of inbreeding. The process of inbreeding is purposefully practiced for generating genetic uniformity of laboratory stocks and to yield stocks for crossing (plant breeding and animal breeding).
Inbreeding is involuntarily produced by keeping small populations (found in zoos).
Inbreeding depression refers to the decrease or loss of the fitness and vigor which is mainly caused due to the inbreeding. To be simpler, 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 inbreeding depression.
This phenomenon occurs in all wild animals, in humans, as well as in plant populations, representing that genetic differences in fitness traits exists both within and among the normal populations. Inbreeding depression plays a significant role in the crop breeding and in the evolution of outcrossing mating systems.
The degree of inbreeding is generally measured by the inbreeding coefficient. Inbreeding depression may or may not be caused by inbreeding as it is the reduction in the fitness of the offspring created by inbred mattings. The inbreeding depression is mainly caused by inbreeding, which is a universal phenomenon that depends on selection, past mutation, and genetic drift.
The result of inbreeding depression is mainly due to the unfavorable fixation of recessive genes in F2 generation. Whereas, heterosis is the phenomenon in which the unfavorable recessive genes of one parent are covered by the favorable dominant genes of another parent.
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 the self-pollination leads to severe depression and exhibits a lethal effect. It is very hard to maintain the line once, after three to four generations and it is due to the loss of vigor and fertility. These are mainly seen in the Alfalfa of a pea family and in carrots.
- Moderate inbreeding depression: Along with the lethal effects, sublethal effects are seen in the offspring’s produced by the self-pollination. There is a considerable decrease in the fertility, as several lines produced are very poor and are lost. Maize, pearl millet, great millet are few examples of plants showing moderate inbreeding depression.
- Low inbreeding depression: A minor proportion of the plants exhibit lethal characteristics. The loss of vigor and fertility is lesser and due to the poor fertility, a line cannot be maintained which results in a reduction in yield due to inbreeding being low or absent. Onion, squash, pumpkin, sunflower are few 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 by heterozygous balance.
Inbreeding depression in Vipera berus (Animal Breeding)
Vipera berus is commonly known 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 Vipera berus from other inhabitants into the isolated population, they produce by recovering a developed portion of viable offspring.
The reason behind the recovery is- A 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, the inbreeding depression is mainly seen only in the smaller population rather than the larger population. Because in the smaller population, when the families 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.
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