animal breeding Heritability and genetic correlations in breeding

Heritability is the proportion of the additive genetic variation to the total variation. Heritability is important because without genetic variation there can be no genetic change in the population. Alternatively, if heritability is high, genetic change can be quite rapid, and simple means of selection are all that is needed. Using an increasing scale from 0 to 1, a heritability of 0.75 means that 75 percent of the total variance in a trait is controlled by additive gene action. With heritabilities this high, just the record of a single individual’s traits can easily be used to create an effective breeding program.

Some general statements can be made about heritability, keeping in mind that exceptions exist. Traits related to fertility have low heritabilities. Examples include the average number of times that a cow must be bred before she conceives and the average number of pigs in a litter. Traits related to production have intermediate heritabilities. Examples include the amount of milk a cow produces, the rates of weight gain in steers and pigs, and the number of eggs laid by chickens. So-called quality traits tend to have higher heritabilities. Examples include the amount of fat a pig has over its back and the amount of protein in a cow’s milk. The magnitude of heritability is one of the primary considerations in designing breeding programs.

Genetic correlation occurs when a single gene affects two traits. There may be many such genes that affect two or more traits. Genetic correlations can be positive or negative, which is indicated by assigning a number in the range from +1 to − 1, with 0 indicating no genetic correlation. A correlation of +1 means that the traits always occur together, while a correlation of − 1 means that having either trait always excludes having the other trait. Thus, the greater the displacement of the value from 0, the greater the correlation (positive or negative) between traits. The practical breeding consequence is that selection for one trait will pull along any positively correlated traits, even though there is no deliberate selection for them. For example, selecting for increased milk production also increases protein production. Another example is the selection for increased weight gain in broiler chickens, which also increases the fat content of the birds.

When traits have a negative genetic correlation, it is difficult to select simultaneously for both traits. For example, as milk production is increased in dairy cows through genetic selection, it is slightly more difficult for the high-producing cows to conceive. This negative correlation is partly due to the partitioning of the cows’ nutrients between production and reproduction, with production being prioritized in early lactation. In the case of dairy cattle, milk production is on the order of 20,000 pounds per year and is increasing. This is a large metabolic demand, so nutrient demand is large to meet this need. Thus, selecting for improved fertility may result in a reduction in milk production or its rate of gain.