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

GENETIC COMPLICATIONS

Heredity and environment

• Most traits of economic importance are controlled by many genes and are greatly influenced by environment also. The environment may alter the traits and mask the real genetic worth of the individuals. For example, an animal with a faster growth rate rose in a faulty environment (deficient diet) and an animal with poor genetic constitution for growth rate but raised in a good environment can be responsible for mistakes in selection. This effect of environment can be responsible for mistakes in selection. However, this effect could be overcome by keeping the stock under selection in a standard and suitable environment wherein the better genotype will be able to express itself fully. Under such conditions, the breeder will have a chance to recognise the differences that are hereditary and thus increase the accuracy of selection.

Genotype and phenotype

• Genotype is animal’s genetic constitution. The genotype remains constant for an animal throughout its life. But phenotype is the result of interaction between the genotype and environment in which the animal is developing. The phenotype, unlike the genotype, changes with time. Hence it affects selection. Selection is done for the genotype, but seldom, it could be assessed correctly. So the genotype is assessed based on phenotype of the individual though it is not accurate. So, for selection to be effective, phenotypic selection should be done at the age when the economic traits are expressed, for e.g., meat animals like sheep, swine and poultry, phenotypic selection should be done at market age. Cows should be selected at the end of first lactation.

Heritability

• Most selection processes are based on phenotypic differences. Though we are selecting on phenotypic basis, our aim is to effect selection on genotypic basis. If the phenotype accurately reflects the genotype, the selection will be quite accurate. But phenotype is not a true indicator of genotype. Heritability of a trait may be defined as that portion of the phenotypic variation that is due to additive gene action. If most of the phenotypic variation is due to environment, progress from selection will be slow. On the other hand, if the additive genetic variation is larger, then the heritability estimate will more accurately predict the genotype. The heritability values are not constant and vary from herd to herd and in the same herd from time to time. Inbreeding for instance increases homozygosity of genes and reduces the hereditary variation. Therefore, heritability will decrease with inbreeding and increase without crossing. In other words, phenotype or individual selection will be more effective in herds and for traits where the heritability is high. Hence, knowledge of heritability of economic traits in livestock is therefore essential for a breeder.

Regression to mean

• The offspring of outstanding parents often have a tendency to regress towards the average of the breed from which they were selected. This is referred to as Galton’s law of filial regression. This is because (i) due to combination of genes; when they reproduce due to segregation and independent assortment of genes, the suitable combination is broken up and the average results and (ii) due to environment; the offspring are brought up in an environment which is much different from that of the parent. If the superiority of the parents is due to lucky combination of genes, little could be done to interfere with the laws of segregation and independent assortment. If the superiority of the parents is due to high percentage of homozygosity of favourable genes, by adopting inbreeding the gene pool could be maintained in the offspring. If the superiority of the parents is due to heterosis i.e., Aa (heterozygous) better than AA or aa (homozygous), it is not possible to control the segregation of genes. So, heterozygous individuals that are superior could be used for market but not for breeding. The environmental part of regression can be levelled out a great deal by keeping the same environment as far as possible from year to year. This is another reason why animals should be tested and selection should be made under conditions similar to one in which their offspring are to perform.

Type of gene action

• Genes act differently in different combinations. This makes accurate selection more difficult. For instance, when “A” is dominant to “a”, AA and Aa individuals who have the same phenotype will be selected with equal preference. But AA will breed true whereas Aa will segregate. But in case of over dominance, Aa will produce larger effect than AA / aa. So in heterozygous condition, selection will not produce desired results. Only crossing of appropriate inbred lines will produce the desired effect. Hence the job of the breeder is to increase the frequency of favourable alleles and to discard the less favourable ones.

Correlation of traits

• Some characteristics are positively correlated, for example, rate of gain in weight and efficiency of gain in swine. Whereas some others are negatively correlated, for example, milk yield and butter fat percentage in dairy cattle. If the desirable traits are positively correlated selection becomes somewhat easier. If you select for one trait it automatically improves the other trait also. When the traits are negatively correlated, selection for one trait will affect the other trait. Hence, knowledge of correlation of different traits will be of great help in avoiding mistakes in selection.

Effect of inbreeding

• It is generally known that a decline in all attributes of vigour usually accompanies inbreeding. Breeders therefore hesitate to practice inbreeding. However, it is necessary to practice inbreeding in order to induce gene segregation and to fix desirable gene combinations. Inbreeding increases prepotency. Regularity of transmission is increased with increased homozygosity, which is obtained only through inbreeding and selection. To avoid depressing effects of inbreeding: choose foundation stock that is superior in production, rigid selection from beginning to offset the possible bad effects of inbreeding on performance and flexible system of mating that permits besides close breeding, mating of best individuals that is controlled breeding.