There is a thick line of connection between science, biology and genetics. Biology is the branch of science and genetics is the sub- branch of biology. Similarly, if we go upward to the further branches, genetics have one major portion that deals with population and all the genetic variations that occur with passing time and this portion is named as population genetics.
Population genetics is one of the important topics in genetics that provides deep insight on the changes in overall population due to genetic factors. This basic concept and idea of his topic is based on Hardy- Weinberg assumptions that are fit for the ideal population. So before going depth about inbreeding it is equally necessary to recapitulate the laws of Hardy- Weinberg so that understanding inbreeding will surely become an easy task. Therefore if these assumptions are to describe in one sentence, allele frequency will remain same and no change will be noticed if there will be no migration, mutation, selection in large population. Along with this, random mating also will help population remain stable without any change in the allele frequency. So these are some of the assumptions given by Hardy- Weinberg on ideal population.
Inbreeding clearly refers to the mating of individual that are closely related. Though the event takes place randomly, but the two individual finally comes up to be the close relatives, i.e. they must have shared common line of ancestry at one point of time. Now, when two closely related individuals are mated, undoubtedly the result t would be increase in homozygosity. Yes, inbreeding increase the proportion of homozygote’s in the population and lowers the chances of heterozygosity. Inbreeding can be estimated with the help of coefficient of inbreeding given by Sewall Wright and is also designated as F. Here, if F=1 it can be said that alleles in every individual comes from their ancestor and if F=0, then no common allele is present in between two or more individuals. The best example of inbreeding can be self- fertilization, where homozygosity increases with every generation and at the end no heterozygote’s can be noticed. Since, heterozygosity is inversely related to inbreeding so the mathematical formula that helps to calculate inbreeding is: He-Ho/He. Here, He is expected heterozygosity and Ho is observed heterozygosity. Hence, if the result of both comes out to be 0 then, undoubtedly as mentioned before no common allele is present between the individuals. Beside this, mathematical calculation the other way round to calculate inbreeding is the construction of pedigree. With the help of pedigree as well inbreeding can be correctly calculated.
Since inbreeding generally give rise to homozygosity, so chances of having recessive deleterious allele in an individual increases. Along with this, in case of human, abortion, early death of newborn, etc can also be noticed due to inbreeding. Along with this, the chances of diseases such as albinism, alcaptoneuria also increases. Beside this, there is one more term that describes inbreeding in terms of the fitness called inbreeding depression. Hence, inbreeding depression means as the homozygosity increases it results in the loss of fitness in the population.
Though inbreeding have many disadvantages but in many cases lots of plus points can be noticed due to inbreeding. Firstly, when natural selection combines with inbreeding then deleterious allele gets eliminated and hence evolution takes place. Secondly, in case of plants good varieties of plants can be generated in numerous numbers through the process of inbreeding that is any desirable trait can be produced through inbreeding and this is also called hybrid vigour. Hybrid vigour is though a very new term but it is explained in two different ways: first is the dominance hypothesis that tells us that the F1 hybrids from two different strains will have more dominant alleles that favor the better trait in the offspring. Secondly, over dominance that tells us that though heterozygosity have both the character of parents and hence is better than homozygosity, its cumulative effect on different loci will result in hybrid voigour.
This way, inbreeding has both good points and the bad points, practice of inbreeding is good but constantly following inbreeding can be somehow deleterious and harmful for the human.