We have estimates of heritability for a number of characters in beef cattle. Some of these are higher than seems reasonable in comparison with the figures obtained from swine and sheep and in view of the probable effects of environmental factors on such characters as final feedlot weight, rate of gain, and economy of gain in the feed lot. More information is needed before these figures can be accepted as generally representative, but at least they indicate that selection should be effective in improving most of the characteristics studied.

Data on heritability of various characters in sheep also indicate that most of the various desirable traits studied can be improved by selection, although selection for such traits as yearling body score, type score at weaning, and condition score at weaning would not lead to rapid progress. The several figures for heritability of skin folds and those for face covering indicate that it should be possible to make rather rapid progress in the elimination of excessive skin folds and covered faces by selection for animals that are smooth and have open faces. One of the figures for heritability of neck folds (8 percent) is low, but it was obtained on breeds that are characterized by relatively few skin folds compared with the Rambouillet, on which the other estimates for this character are based.

The heritability of a trait is one of the most important factors to consider in deciding upon the breeding plan that is most apt to be successful in bringing about improvement in that trait. If the heritability of the desired trait is high, the best method of breeding to bring about improve-men will be the mating of animals possessing greatest development of the desired trait, little use being made of information on pedigrees and relatives. If heritability is low, the breeder is more apt to make progress if he uses information on pedigrees and collateral relatives and information he gets from progeny tests in deciding which animals to use for breeding. Also, if heritability is low, it is generally advisable to make relatively little use of inbreeding other than the inbreeding that is needed to make families distinct from each other or to make full use of the progeny test.

Heritability of a trait may be due to additive effects of genes, or there may be variations in hereditary effects owing to epistatic effects of genes, or both types of effects may be present. The difference between these two types of effects may be illustrated by supposing that two dominant genes, A and B, located at different points on a chromosome or on different chromosomes, have values of 6 and 4, respectively, when one occurs without the other, insofar as they affect a certain trait. If their effects are strictly additive, the combined value of the two is 10, if both are present in an animal. If, however, the value is 12 when they occur together, the effects are not strictly additive, and the extra value resulting from the interaction of the two is called an epistatic effect. If additive effects of genes are low, but heritability of a trait appears to be fairly high because of epistatic effects, inbreeding to develop lines that are distinct from each other, selection of the outstanding lines, crossing these lines, and developing new ones from the more favorable crosses is the procedure that appears most likely to be effective.

There is need for much additional information on heritability of various economically important traits in livestock, and on the nature of the effects of genes that control the development of these traits, before it will be possible to prescribe methods of breeding that will be most effective in all situations with which breeders are confronted. However, sufficient information is available to indicate some of the advantages and limitations of inbreeding and crossing, and the possibilities of developing new types from crossbred foundations. These problems are discussed later.