Inbreeding Depression

“Inbreeding depression is the reduction in the biological fitness of the individuals of a population as a result of inbreeding. “

What is Inbreeding Depression?

Inbreeding depression refers to the decrease or loss of fitness and strength which is mainly caused due to 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 significant role in crop breeding and in 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. The hermaphrodite species C.elegans exhibit lower degrees of inbreeding depression. 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.

Also Read: Plant Breeding

Inbreeding Depression in Plants

Inbreeding depression in Plants

Onion, carrot, maize, sunflower, etc. are a few examples of plants showing inbreeding depression. They reproduce 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 lead to 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 offsprings produced by self-pollination. There is a considerable decrease in fertility, as several lines produced are very poor and lost. Maize, pearl, millet, great millet are a few 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 a 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 with heterozygous balance.

Inbreeding depression in Vipera berus (Animal Breeding)

Inbreeding depression in Vipera berus

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 offsprings 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 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 mainly seen only in smaller populations rather than 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.

Also Read: Types of Pollination

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