In this article we will discuss about:- 1. Methods of Inbreeding 2. Genetic Effects of Inbreeding 3. Inbreeding Depression 4. Practical Applications.
The process of mating of individuals which are more closely related than the average of the population to which they belong, is called inbreeding. For example, parthenogenesis in animals and apomixes and self-fertilization in plants are the most extreme types of inbreeding.
Inbreeding in self-fertilizing pea plants was a real advantage to Mendel in his studies which provided pure lines of pea plants for his hybridization experiments. The term ‘pure line’ was coined by W. Johannsen in 1903 for the true breeding, self-fertilized plants.
(For obtaining pure lines Johannes performed classical crosses on common garden bean plant or Phaseolus vulgaris). Pure line population is the one which breeds true when selfed without producing any genetic variability in the progeny.
Methods of Inbreeding:
In plants ova fertilized by the pollen of either the same plants (in case of bisexual plants) or of the other plant of the same genotype (in case of unisexual as well as bisexual plants), is called self-fertilization.
However, in bisexual plants numerous structural and functional adaptations have been recorded which help plants with bisexual or hermaphrodite flowers avoid self-fertilization.
Normally, inbreeding is affected by restrictions in population size or area which brings about the mating between relatives. Since close relatives have similar genes because of common heritage, inbreeding increases the frequency of homozygotes, but does not bring about a change in overall gene frequencies.
Thus, a mating between two heterozygotes as regards two alleles, A and a will result in half of the population, homozygous for either gene A or a and half of the population heterozygous like the parent but the overall frequencies of A and a remain unchanged:
Aa × Aa
1AA : 1Aa
Thus, inbreeding brings about the recessive gene to appear in a homozygous stale (aa). Once a recessive allele is in a homozygous state, natural selection can operate upon the rare recessives. Artificial selection is also possible as the homozygous recessives are phenotypically differentiated from the dominant population.
The inbred pedigrees can be depicted as follows:
Here, B and C are full sibs, i.e., have common parents.
This pedigree can also be represented by the following arrow diagram:
1. Coefficient of Relationship (R):
Coefficient is expression of the amount or degree of any quality possessed by a substance. It is also the degree of physical or chemical change normally occurring in that substance under stated conditions. The coefficient of relationship (R) characterises the percentage of genes held in common by two individuals due to their common ancestory.
Each individual gets only a half of his genotype from one of his parent, each arrow in the above arrow diagram represents a probability of half. The sum (Σ) of all pathways between two individuals through common ancestors is the coefficient of relationship and is represented by R:
(i) RBC = the coefficient of relationship between the full sibs B and C and is calculated as follows:
i.e., individuals B and C contain ½ × ½ = ¼ of their genes in common through ancestor D.
(ii) i.e., individuals B and C contain ½ × ½ = ¼ of their genes in common through ancestor E.
(iii) The sum of these two pathways, the coefficient of relationship, between the full sibs B and C = ¼ + ¼ = ½ = 50 per cent.
2. Inbreeding Coefficient:
In a diploid organism, each gene has two alleles occupying the same locus. They are called identical genes if they have descended from the same gene; such genes are homozygous at the locus.
Such a homozygosity is also caused when two alleles in a diploid organism have not descended from the common gene but the alleles of identical origin are brought together through mating between first cousins. Such alleles are called similar alleles.
The fine difference between these two types of alleles becomes clear by the following chart:
The probability that the two alleles in a zygote are identical by descent, i.e., are the replication product of the same gene of an ancestor is measured by the inbreeding coefficient (F) and is calculated as follows:
1. If the parents B and C are full sibs, i.e., B and C parents are 50 per cent related, the inbreeding coefficient of individual (A) can be calculated by the equation FA = ½ RBC, where RBC is the coefficient of relationship between the full sib parents (B and C) of A.
2. If the common ancestors are not inbred, the inbreeding coefficient is calculated by the equation:
F = Σ (½) n1+n2+1
where n1, is the number of generations (arrows) from one parent back to the common ancestor and n2 is the number of generations from the other parent back to the same ancestor.
3. In case the common ancestors are inbred, the inbreeding coefficient is calculated as follows:
F = Σ (½) n1+n2+1(1+Ancestor)
4. The coefficient of inbreeding is also calculated by counting the number of arrows connecting the individual through one parent back to the common ancestor and back again to his other parent by the following equation:
F = Σ (½)n (1+FA)
n = number of arrows which connect the individual through one parent back to the common ancestor and back again to his other parent. FA is the inbreeding coefficient of the common ancestor. For example, the inbreeding coefficient for A in the following arrow diagram can be calculated by following method:
B and C are the parents of A. There is only one pathway from B and C and that goes through ancestor E. Ancestor E is inbred, because its parents (G and H) are full sibs and are 50 per cent related.
The inbreeding coefficient can be calculated as follows:
FE = ½ RGH (R = the coefficient of relationship between the full sibs G and H)
or FE = ½ (0.5) = 0.25
FA = Σ (½)n (1 + FE (ancestor))
or FA = (½)3 (1 + 0.25) = 0.156
3. Panmixis (Random Mating):
If the breeder assigns no mating restraints upon the selected individuals, their gametes are likely to randomly unite by chance alone. This is commonly the case with outcrossing (non-self-fertilizing) plants. Wind or insect carry pollen from one plant to another in essentially a random manner.
Even livestock such as sheep and range cattle are usually bred panmicticly. The males locate females as they come into heat, copulate with (“cover”) and inseminate them without any artificial restrictions as they forage for food over large tracts of grazing land. This mating method is most likely to generate the greatest genetic diversity among the progeny.
4. Assortative and Disassortative Matings:
In sexually reproducing organisms, the most rapid inbreeding system is that between brothers and sisters who share both parents in common. This type of mating is called full-sib mating and produces inbreeding coefficient of 25 per cent in the first generation of inbreeding (F2 of Mendel).
This rate is reduced in succeeding generations since some of the alleles are now already identical. Within 10 generations, full-sib matings can produce an inbreeding coefficient of 90 per cent. The other inbreeding systems are half-sib mating, parent-offspring mating, third-cousin mating and so on.
All these inbreeding systems are called genetic assortative matings since the parents of each mating type are sorted and mated together on the basis of their genetic relationship. Such a breeding method tends to increase the inbreeding coefficient.
The assortative mating is also of the phenotypic type, i.e., the mating between two like phenotypes, two like dominant phenotypes or between two like recessive phenotypes. If assortative selective mating is continued for many generations, the heterozygotes are eliminated and the resulting population consists of homozygous dominants and homozygous recessives.
If more than one locus is considered at a time, the rate of homozygosity achievement will be slower than for one locus. This is so because now the kind of heterozygotes produced will be more combinations of different loci, e.g., Aa BB, AA Bb, … ) and eliminating these will need more number of generations.
Disassortative mating refers to the mating of unlike phenotypes and genotypes and tends to maintain heterozygosity, as in the case of mating between unlike sexes. This preserves the dissimilarities both genetic as well as phenotypic.
In primitive organism, sexual differences arose at a single gene locus, i.e., one sex was homozygous and the other heterozygous for that locus, and the disassortative matings were the matings between an homozygous and an heterozygous individual for sex locus.
Disassortative mating also results from dichogamy, (Dichogamy = producing mature male and female reproductive structures at different times); self-sterility in plants in which the mating of like phenotypes (inbreeding) is not possible and fertilization between plants with different genotype is favoured. This maintains heterozygosity within a diploid breeding population.
5. Line Breeding:
It is a special form of inbreeding Utilized for the purpose of maintaining a high genetic relationship to a desirable ancestor.
D possesses 50 per cent of B’s genes and transmits 25 per cent to C. B also contributes 50 per cent of his genes to C. Hence, C contains 50 per cent +25 per cent= 75 per cent B genes and transmits half of them (37.5 per cent) to A. B also contributes 50 per cent of his genes to A. Therefore, A has 50 per cent + 37.5 per cent = 87.5 per cent of B’s genes.
Genetic Effects of Inbreeding:
The continuous inbreeding results, genetically, in homozygosity. It produces homozygous stocks of dominant or recessive genes and eliminate heterozygosity from the inbred population.
For example, if we start with a population containing 100 heterozygous individuals (Aa) as shown in figure, the expected number of homozygous genotype increasing by 50% due to selfing or inbreeding in each generation.
Thus, due to inbreeding in each generation the heterozygosity is reduced by 50% and after 10 generations we can expect the total elimination of heterozygosity from the inbred line and production of two homozygous or pure lines.
But, because a heterozygous individual possesses several heterozygous allelic pairs, we can conclude that inbreeding will operate on all genes loci to produce totally pure or homozygous offspring’s.
In human beings if inbreeding continued over a number of generations, it would results in increasing homozygosity, but somewhat slowly.
In a heterozygote, the inbreeding increases the probability of homozygosity of deleterious recessive alleles in an inbred population. In other words, one of the consequence of inbreeding is a loss in vigour (i.e., less productive vegetatively and reproductively) which commonly accompanies an increase in homozygosity. This is called Inbreeding depression.
Inbreeding depression is found to occur due to following four features of inbreeding:
(1) Increase in frequency of homozygotes,
(2) Increase in variability between different inbred families,
(3) Reduction in value of quantitative character in the direction of recessive values, and
(4) The dependence of this reduction in value upon dominance.
If this inbreeding effect is multiplied for many genes at many loci, there may be a large reduction in value for many traits, including those that affect fitness and survival. In com (maize) for example, E.M. East (1908) and G. H. Shull (1909) studied the effects of inbreeding for 30 generations of inbreeding and found independently, that the yielding ability in these lines finally reduced to about one-third of the open-pollinated variety from which these samples were derived.
Both of these authors draw the following important conclusions:
(1) A number of lethal and sub-vital types appear in early generations of selfing.
(2) The material rapidly separates into distinct lines, which become increasingly uniform for differences in various morphological and functional characteristics.
(3) Many of the lines decrease in vigour and fecundity until they cannot be maintained even under the most favourable culture conditions.
(4) The lines that survive show a general decline in size and vigour.
Figure 52.4 shows the decline in size and vigour due to inbreeding in maize; here, the inheritance of two quantitative traits namely plant height and grain yield of three lines are shown for 30 generations of inbreeding. It can be noticed that fixation for plant height occurred after five generations of inbreeding. However, yield continued to decline for at least 20 generations until it reached one-third that of open-pollinated variety from which they were derived.
Despite this conspicuous decline, maize was found more tolerant to inbreeding than some organisms where few strains survive two or three generations of inbreeding, e.g., alfalfa and onions.
In alfalfa, upon selfing many sub-vital and lethal types appear and the rate of decline of general vigour and productivity is alarming. The very small number of lines which survive give a greatly reduced forage yield.
But onions (a normally cross- pollinated species) are quite tolerant to inbreeding, i.e., they show much less depression in vigour due to inbreeding than alfalfa and maize. Carrot is another cultivated species in which inbreeding leads to loss in vigour and production.
The following cross- pollinated plants are found to be fairly tolerant to inbreeding: sunflowers, rye, timothy, smooth broom-grass and orchard grass. In certain self- pollinated species and normally cross-fertilizing species such as cucurbits, inbreeding is found to be continued indefinitely without any ill effect.
In most animals, inbreeding is found to have less remarkable effects on vigour. For example, in rats continuous brother-sister matings were performed for 25 generations, but no drastic deterioration was detected. In Drosophila, inbreeding usually results in a rapid loss of vigour, but some strains compare favourably with outbreed populations after long continued inbreeding.
However, in certain breeds of cattle, intensive inbreeding has led to an unfortunate condition; for example, exhaustive inbreeding and selection of beef cattle breed (Hereford) produced dwarf calves of low economic value.
These calves show characteristic head and body features of the brachycephalic dwarfism (i.e., the characteristic short, broad head, extra long lower jaw, bulging forehead, out of proportion abdomen and short legs).
Breeding data indicate that a basic recessive gene is necessary for dwarfing, but additional modifier genes have been postulated to account for the different types of dwarfs.
Practical Applications of Inbreeding:
The correlation of inbreeding and homozygosity exhibits how inbreeding may cause deleterious effects. As we already know that in a heterozygous individual, the harmful recessive alleles remain masked by their normal dominant alleles.
If a hetetozygous individual undergoes inbreeding for various generations, there will be equal chances of homozygosity for dominant as well as recessive alleles. In homozygous condition, recessive alleles will be able to express their deleterious phenotypic effects on an individual. On the other hand, the homozygosity for dominant alleles have equal opportunity to express their beneficial phenotypic effects on inbred races.
The practical applications of inbreeding are following:
1. Because inbreeding cause homozygosity of deleterious recessive genes which may result in defective phenotype, therefore, in human society, the religious ethics unknowingly and modem social norms consciously have condemned and banned the marriage of brothers and sisters. Further, the plant breeders and animal breeders too avoid inbreeding’s in the individuals due to this reason.
2. The inbreeding because, results in the homozygosity of dominant allele, therefore, it is a best means of mating among hermaphrodites and self-pollinating plant species of several families. The animal breeder have employed the inbreeding to produce best races of horses, dogs, bulls, catties, etc.
The modern race horses, far example, are all descendants of three Arabian stallions imported into England between 1689 and 1730 and mated with several local mares of the slow, heavy type that had carried the medieval knights in heavy armour.
The fast runners of F1 were selected and inbred and stallions of the F2 appear as beginning points in the pedigrees of almost all modem race horses. This sort of inbreeding in also called line breeding which has been defined as the mating of animals in such a way that their descendants will be kept closely related to an unusually desirable individual.
Similarly, merino sheep are widely known as fine wool producers. They are the result of about 200 years of inbreeding. This strain was being developed in Spain in the 17th century by stock raisers.
They observed that the ancestors of the present day merino sheep had two coats of wool, one composed of long, coarse fibres arising from primary follicles, and a second coat composed of short fine wool arising from clusters of secondary follicles.
Intensive artificial selection was maintained for animals with more uniform production of fine wool and a lesser amount of coarse wool. For a time, Spain had a monopoly on the valuable merino sheep.
When France invaded Spain, merino sheep were moved to France where they were maintained and eventually distributed to other parts of the world. Merino sheep were taken to South Africa and in 1796 they were introduced into Australia which has since become the world’s largest producer of fine wool.