Cross Breeding

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Cross Breeding

Cross Breeding
Mating of individuals from entirely different races or even different species is called cross breeding. This represents the most extreme form of out breeding that is possible among animals. Cross breeding produces sterile hybrids in comparison to normal outbreedings.
Example- The mule is a heterotic individual which had been produced by cross breeding of a male donkey (Equus asimus, 2n=62) with the female horse (Equus caballus, 2n=64). It displays a conspicuous vigorosity and because of this it has served mankind as a patient beast of burden since time immemorial.

Genetical Basis of Heterosis
The genetical basis of heterosis is still a subject of controversy and following two theories have been propounded to explain it:
1. Over dominance theory of heterosis-The over dominance theory or physiologic stimulation hypothesis of Shull (1874-1954) considers heterozygosity to have stimulating effect on the heterosis of hybrid individual. Heterozygosity itself was thus considered to be the controlling factor.
Example-If we suppose that four gene loci are contributing to a quantitative trait, homozygous recessive genotype contribute 1 unit to the phenotype, heterozygous genotype contribute 2 units to the phenotype and homozygous dominant genotypes contribute Ii units. Then, the results can be represented as follows:

2. Dominance theory of heterosis- The dominance theory of heterosis holds that increased vigour and size in a hybrid is due to combination of favourable growth genes by crossing two inbred races. In other words, the hybrid vigour is a result of action and interaction of dominant or fitness factors or cumulative (polygenic) effect of dominant genes.
Example-If we suppose, that a quantitative trait is governed by four genes. Each recessive genotype contributes on unit to the phenotype and each dominant genotype contributes two units to the phenotype. A out cross (out breeding) between two inbred lines can produce more heterotic F1, individuals than the parents. in the following manner:

Parents:	AABBccdd	x	aabbCCDD
Phenotypic value:	11/2+11/2+1+1=5	↓	1+1+11/2+11/2=5
F₁		AaBbCcDd
		2+2+2+2=8

Parents:	AAbbCCdd	x	aaBBccDD
Phenotypic value:	2+1+2+1=6	↓	1+2+1+2=6
F₁		AaBbCcDd
		2+2+2+2=8

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