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.
Conclusion
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.
FAQs on Inbreeding Depression
1. What is inbreeding depression as per the CBSE Class 12 syllabus?
Inbreeding depression is the reduction in the biological fitness and vigour of a population resulting from the mating of related individuals (inbreeding). This continuous inbreeding, especially close inbreeding, increases homozygosity. As a result, harmful or deleterious recessive alleles, which are normally masked, have a higher chance of being expressed, leading to decreased fertility, lower productivity, and reduced survival rates.
2. What is the primary genetic reason behind inbreeding depression?
The primary genetic cause of inbreeding depression is the increase in homozygosity for recessive alleles. In any population, individuals carry hidden harmful recessive alleles. When closely related individuals mate, there is a significantly higher probability that their offspring will inherit two copies of the same harmful allele. This expression of deleterious traits, which would be masked in a heterozygous individual, leads to a decline in the overall fitness of the offspring.
3. How does inbreeding depression affect agricultural plants? Provide examples.
In agricultural plants, inbreeding depression manifests as a significant loss of vigour and fertility, which can impact crop yield. The severity varies among species:
- Severe Depression: In plants like Alfalfa and carrots, continuous self-pollination leads to a rapid loss of vigour and fertility, making it difficult to maintain the breeding line after a few generations.
- Moderate Depression: In crops like maize and pearl millet, there is a considerable decrease in fertility, and many resulting lines are too weak to be productive.
- Low Depression: Plants like onions, squash, and sunflowers show a lesser loss of vigour and fertility upon inbreeding.
4. How can breeders and farmers manage or overcome inbreeding depression?
The most common strategy to overcome inbreeding depression is through outcrossing. This involves mating selected individuals from the depressed population with unrelated superior individuals of the same breed (but having no common ancestors) or from a different breed. This practice, known as out-crossing, introduces new genes into the population, restores heterozygosity, and masks the effects of harmful recessive alleles, thereby restoring fertility and vigour.
5. Why is inbreeding depression a greater concern in small, isolated animal populations?
In small and isolated populations, the gene pool is limited, and individuals are more likely to be related. This leads to a higher frequency of mating between relatives over generations. Consequently, there's a rapid increase in homozygosity across the population, which unmasks deleterious recessive alleles. The example of the European viper (Vipera berus) in an isolated habitat showed a higher number of stillborn and deformed offspring until new individuals were introduced, which restored the population's viability.
6. How does inbreeding depression contrast with heterosis (hybrid vigour)?
Inbreeding depression and heterosis are essentially opposite phenomena.
- Inbreeding Depression is the loss of fitness due to increased homozygosity from mating related individuals.
- Heterosis (or hybrid vigour) is the increase in fitness, size, and productivity of a hybrid offspring that results from crossing two different, often inbred, parent lines. The superiority of the hybrid is due to increased heterozygosity, which masks deleterious recessive alleles and can lead to superior gene combinations.
7. Does inbreeding depression occur in humans?
Yes, inbreeding depression can and does occur in humans. It is observed in populations or communities where there is a high prevalence of consanguineous marriages (marriages between close relatives). This practice increases the likelihood that offspring will inherit two copies of a recessive allele for a genetic disorder, leading to a higher incidence of certain inherited diseases and reduced overall biological fitness compared to the general population.
