Heritability

The development of an animal depends upon its inherited make-up and the environment in which it lives. Improvements in heredity are permanent, except those that result from particular combinations of genes, the determiners of heredity, and that disappear when the genes recombine. Improvements in environment must be provided again for each succeeding generation.

The heritability of a trait is actually a measure of the observed variation in a group of animals that is caused by differences in heredity. Estimates of heritability are based on the degree that related animals resemble each other more than less closely related or unrelated animals.

These estimates are applicable primarily to characters or traits in which development depends upon many genes.

Considerable information has accumulated in recent years on the heritability of various characters in livestock. The information helps the breeder because it indicates the progress that can be made by selection and the plan of breeding that is likely to be most effective. Practically all the information on heritability of economically important traits in livestock has been obtained during the last decade. A summary of it is given in the accompanying tables.

There are, of course, variations in the estimates of heritability, and many apparent discrepancies. There are several reasons. Errors may occur in sampling, particularly in studies based on small numbers of animals, so the results are not representative. Variations in environment may be correlated for certain kinds of relatives. For example, data may have been collected over a period of years in which gradual changes in feed or management occurred. Thus, both dams and their progeny, raised at various times during this period, may have been exposed to an environment better or poorer than the average. Such environmental contributions to likenesses between relatives are difficult to measure. Another factor that may affect estimates of heritability is the mating system. A different approach is required to obtain a reasonably accurate estimate of heritability in an inbred population than in one where random mating has been practiced, a factor that has not been taken into account in some of the studies. In others, the mating system or the amount of inbreeding may have deviated more (or less) from random than the investigator supposed.

Data on the heritability of weights of swine at various ages, and on the heritability of rate of gain (tables 1 and 2) indicate that the estimates of heritability increase as pigs grow older. Therefore, selection for maximum weight should be most effective if practiced at 180 days, rather than at earlier ages. Heritability of weight at 180 days approximates 30 percent. This means the breeder should expect to make about 30 percent of the progress he "reaches for" in selection. For example, if he selects for parents of the next generation animals that weigh 20 pounds above the average of his stock at 180 days, their offspring should be expected to weigh about 6 pounds more than the average of offspring from parents picked at random from the same stock.

Litter size at birth is not so highly inherited as weight at 180 days. The unweighted average of the eleven estimates given (in table 3) is 19.2 percent. This means that the breeder can expect to realize about one fifth of the progress in litter size that he reaches for in selecting the parents of the next generation. If selection for litter size is based only on sows, no attention being given to boars in this respect, then the progress will be only about one-half of one-fifth. The estimates of progress through selection are based on the assumption that all the heritability for each trait is due to additive effects of genes. If a portion of it is due to epistatic effects (interactions between different pairs of genes), the effectiveness of selection would be somewhat less.

Estimates of heritability for a number of other characters in swine have also been obtained (table 4), but only one study has been made of each of these characters, except type scores, for which there are two figures. The estimate for type score (between strains) is exceptionally high, 92 percent. This was obtained on a combination of three populations representing large, medium, and small-type Poland China swine, and indicates that the differences between these three strains were largely due to. heredity. The estimate of 38 percent for type score (within strains) is the one that indicates the approximate amount of progress that might be made in selecting for type within one of the strains or in a relatively uniform breed. Most of the estimates of heritability for other traits are sufficiently high to indicate that fairly rapid progress can be made by selection. The estimate for economy of gain is quite low, and, if representative, indicates that little improvement could be made per generation as a result of selection for this trait. Further studies are necessary to establish, within reasonably accurate limits, the extent to which these and other important traits of swine are inherited.