AGB 211: Livestock and Poultry Breeding (2+1)

Heterosis

HETEROSIS OR HYBRID VIGOUR

Crosses of animals from different strains or lines of the same breed, from different breeds or from different species, result in offspring whose level of production is above that of the average of the parents. The increased production may be due to increased fertility, increased pre and post natal viability, faster and more efficient growth, improved mothering ability etc. The increased level of performance as compared to the average of the parents is known as heterosis or hybrid vigour. The heterosis can be either positive or negative. Heterosis is the phenomenon in which progeny of crosses between inbred lines or purebred populations exceed the average of the two parental populations.

It is just the opposite of inbreeding depression.

Heterosis can be measured by using the formula

Heterosis (H) = [ (Mean of F₁ offspring) - (Mean of parents) /Mean of Parents ] x 100

Example: The mean litter size at weaning in pigs

Breed A = 7.0

Breed B = 8.0 Mean of A & B = 7.5

F₁ offspring = 8.5

Heterosis    = (8.5 - 7.5) / 7.5 = 1.0/7.5

= 0.13333

= 13.33%

Various types of heterosis are recognised in breeding.

• Parental heterosis (maternal and paternal)
• Individual heterosis referring to the non-parental performance.
• Heterosis is due to non-additive gene action.

Genetic basis of heterosis

The theories put forward to explain heterosis are

• Dominance Theory : It postulates that the parental lines are homozygous dominant for different loci – when crossed produces progeny with dominant gene at all loci.
• Overdominance Theory : It postulates that the heterozygote is superior to either homozygotes (parents).
• Epistasis Theory : It postulates that gene interactions are responsible.

But in practice the heterosis is due to combination of dominance, overdominance and epistasis in any proportion. However, the contribution of epistasis to heterosis is negligible in crossbred of domestic animals.

Generally all the quantitative characters are governed by many genes and no animal is likely to carry all of them in homozygous dominant state. In living organisms, dominant genes are more often favourable than the recessive genes. Crossing of two different lines or breeds has a greater chance of contributing different dominant genes to the progeny.

Since the offspring carries more dominant genes than the parents, it will be more vigourous or productive. All the recessives (aa bb dd ee) except ‘cc’ are masked by the dominant alleles. The degree of heterosis depends on the no. of dominant genes present in the crossbred individual. Maximum heterosis could be obtained if animals carrying all desirable homozygous dominant genes are used for crossing. It would never be possible to have such animals. Eg. Two animals heterozygous for ‘n’ pairs of genes can produce 3ⁿ types of offspring. If only seven pairs of genes are heterozygous, 3⁷ or 2187 types of offspring. If 10 pairs of genes are heterozygous, 3¹⁰ or 59049 types of offspring are possible.

As the quantitative traits are polygenic in nature and the animals produce only a few offspring, it is not possible to produce animals with perfect combination even after many generations of selection. The chance is further reduced by other genetic factors like undesirable recessives, linkage between desirable and undesirable genes and by non-genetic factors like environment.

Formulae H_F1 = dy² and H_F2 = 1/2 dy²