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

GENETIC BASIS OF HETEROSIS

• Heterosis is caused by heterozygosity of genes involving non-additive effects, which mainly includes dominanace, over dominance and epistasis.

Dominance

• When several pairs of genes control one trait, one breed could be homozygous dominant for several pairs and homozygous recessive for another pair (AA BB CC dd) and another breed could be homozygous recessive for respective several pairs and homozygous dominant for respective another pair(aa BB CC DD). Assume that the recessive genotype contributes 1 unit and dominant genotype contributes 2 units of phenotypic values. If these two breeds are crossed:

• This hybrid will be superior to either parent because of presence of at least one dominant gene in all pairs of genes which affect the particular trait.

Over dominance

• For some pairs of genes, the heterozygotes may be more vigorous than either of homozygotes. Here heterozygosity produces hybrid vigour. Consider the same illustration given for dominance producing heterosis. Assume that recessive, heterozygous and homozgous genotypes contribute 1, 2 and 1.5 units of phenotypic values.

• The F1 hybrid generation, phenotypic variability is generally much less than that exhibited by the inbred parental lines or strains or breeds. This shows that heterozygotes are less influenced by environmental factors than the homozygotes. This phenomenon is termed as “buffering”, which means that the organisms’ development is highly regulated by genetics. Another term often used in this connection is “homeostasis”, which means the steady stse in the development of the organism within a normal range of environmental fluctuations.

Epistasis

• To a lesser degree, interallelic interaction or epistasis can account for heterosis. In dominance and over dominance, the heterosis is due to the interaction of genes that are alleles. In epistasis, the interaction is between pairs of genes that are not alleles.

• Contribution of AaBb results in an interaction such that the presence of both A and B gives a phenotype larger or in other words more desirable than would be expected from average phenotypes of AAbb or aaBB.

Application of heterosis in animal breeding

• Not all traits in farm animals are affected to the same degree by heterosis. Those traits expressed early in life, such as survival and growth rate to weaning seem to be affected most. Feed-lot performance as measured by rate and efficiency of gain after weaning is moderately affected. Heterosis has very little effect on carcass traits. Traits, which show the greatest degree of heterosis are the same ones which show the greatest adverse effects when inbreeding is practiced. Highly heritable traits seem to be affected very little by heterosis; whereas, those which are lowly heritable are affected to a greater degree. For example, fertility and litter size in swine (heritability is 15 to 17%).
• The degree of heterosis depends on degree of genetic diversity of the parents. Therefore, heterosis will be higher when breeds are crossed than lines within the breeds are crossed. Crossing breeds having greater differences in genetic backgrounds should give more heterosis than crossing breeds having similar genetic backgrounds. This is because unrelated parents are less likely than related parents to be homozygous for the same pairs of genes.
• Heterosis is much employed to produce commercial stock where the individual merit is promoted, but the breeding value is lowered.
• The successful exploitation of heterosis depends upon how superior the crosses are over the purebreds and whether it is worth considering the lowering the breeding value of the individual and cost of replacement of purebred stock.
• For these reasons, it is commonly practiced in poultry, swine and sheep where the fertility is high and the cost of replacement of purebred stock is necessary