Inbreeding Depression, Effects of inbreeding


Inbreeding Depression

Inbreeding or consanguineous mating is mating between individuals related by descent or ancestry.

When the individuals are closely related, e.g., in brother-sister mating or sib mating, the degree of inbreeding is high.

The highest degree of inbreeding is achieved by selfing.

The chief effect of inbreeding is an increase in homozygosity in the progeny, which is proportionate to the degree of inbreeding

. The degree of inbreeding of an individual is expressed as inbreeding coefficient (F).

The degree inbreeding is proportional to degree of homozygosity.

Inbreeding depression may be defined as the reduction or loss in vigour and fertility as a result of inbreeding.

Inbreeding depression = F1-F2 / F1 x 100


Inbreeding depression has been recognized by man for a long time. Marriages between closely related individuals has been prohibited since early time in many societies. Because people are aware of the harmful effects of such marriages in the progeny. A systematic observations on effect of inbreeding started during 17th century when inbreeding became a common practice in cattle breeding.

In 1876, Darwin published his book on cross and self fertilization in vegetable kingdom. He concluded that progeny obtained from self fertilization were weaker than those obtained from out crossing.

Darwin also reported the results from his experiments on self and cross fertilization in maize for the first time.

East (1908) and Shull (1909) independently showed the effect of inbreeding depression while working in maize. Subsequently scientists reported inbreeding depression in other crop plants.

It has become clear that in cross pollinated crops and in asexually propagated species inbreeding has harmful effect which are severe.

Effects of inbreeding

Inbreeding is accompanied with a reduction in vigour and reproductive capacity i.e. fertility. There is a general reduction in the size of various plant parts and in yield. In many species, harmful recessive alleles appear after selfing; plants or lines carrying them usually do not survive. The different effects of inbreeding are :

1. Appearance of Lethal and Sublethal Alleles : IB results in appearance of lethal; sublethal and subvital characters. Eg : Chlorophyll deficiencies, rootless seedlings, flower deformities – They do not survive, they lost in population.

2. Reduction in vigour : General reduction in vigour size of various plant parts.

3. Reduction in Reproductive ability : Reproductive ability of population decreases rapidly. Many lines reproduce purely that they can not be maintained.

4. Separation of the population into distinct lines : population rapidly separates into distinct lines i.e. due to increase in homozygosity. This leads to random fixation of alleles in different lines. Therefore lines differ in genotype and phenotype. It leads to increase in the variance of the population.

5. Increase in homozygosity : Each lines becomes homozygous. Therefore, variation within a line decreases rapidly. After 7-8 generations of selfing the line becomes more than 99% homozygous. These are the inbreds. These have to be maintained by selfing.

6. Reduction in yield : IB leads to loss in yield. The inbreds that survive and maintained have much less yield than the open pollinated variety from which they have been developed.

Degrees of inbreeding depression

Inbreeding depression may range from very high to very low or it may even be absent. The ID is grouped into 4 categories.

1. High inbreeding depression : Eg : alfalfa and carrot show very high ID. A large proportion of plants produced by selfing show lethal characteristics and do not survive.

2. Moderate inbreeding depression : Eg : Maize, Jowar and Bajra etc. show moderate ID. Many lethal and sublethal types appear in the selfed progeny, but a substantial proportion of the population can be maintained under self-pollination.

3. Low inbreeding depression : Eg : Onion, many Cucurbits, Rye and Sunflower etc. show a small degree of ID. A small proportion of the plants show lethal or subvital characteristics. The loss in vigour and fertility is small ; rarely a line cannot be maintained due to poor fertility.

4. Lack of inbreeding depression : The self-pollinated species do not show ID, although they do show heterosis. It is because these species reproduce by selffertilization and as a result, have developed homozygous balance.

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