Principles of Genetics and Breeding

13.1.7.1. Cross breeding (Intraspecific crossbreeding)

Mating of individuals from entirely different races, strains, varieties or even different species is called cross breeding. This represents the most extreme form of outbreeding that is possible among animals. One of the most successful and wide spread methods of selective breeding in plant species consists of cross breeding between different strains or lines of the same species, and cultivating the crossbred product.

Cross breeding allows the emergence of new phenotypes and genotypes in the second generation which are different from the parental lines.

Cross breeding and inbreeding bring about changes in genotype frequencies, but not in gene frequencies in the population.

“Inbreeding causes an increase in the frequencies of homozygous genotypes and a decrease of heterozygous genotypes,” and this is reversed by crossbreeding.

The magnitude of these effects depends on the dominance relations and the frequencies of the genes concerned. “If the genes that increase the value of the character are dominant over the alleles that reduce the value then inbreeding will result in a reduction of the population mean.”

Inbreeding, other than the intentional production of inbred lines is often practiced unintentionally. Restricting the effective size of the breeding herd to the minimum required for adequate fry production may result in inbreeding, especially in smaller farms.

Crossbreeding, on the other hand is always an intentional process, either by periodically bringing in “new blood” or through scientifically planned schemes.

Selection combined with cross breeding, aims to stabilize those genotypes which have useful characteristics for aquaculture, to produce new varieties of fish.

The following are some examples of cross breeding and selection carried out in fishes.

Intraspecific crossbreeding (crossing of different strains) often increase growth of aquacultured species. Only a few commercially aquacultured species have been improved by cross breeding. Variable levels of crossbreds showing heterosis for growth rate have been obtained in the channel catfish; rainbow trout; common carp and the Pacific oyster. Heterosis was also found in survival, disease resistance and reproductive traits.