Difference Between Animal Breeding and Plant Breeding
The majority of economically important traits in domestic animals are physiologically complex, and they result from the action of many pairs of genes. Since the phenotypic expression of these genes is strongly influenced by the environment, one of the most difficult problems facing the animal breeder is to make proper allowance for the environment of animals so that he can evaluate their inherited traits.
To plan a breeding program, breeders consider alternative criteria of selection and devise plans for improving the animals’ genotypes. First, the breeder must decide what traits he wishes to improve. They may be growth rate, milk production, number of eggs laid, wool production, or some combination of traits. Then, to produce the next generation, he selects the animals he thinks are superior for the desired trait or traits. The criteria of selection differ from species to species and even from trait to trait within a species.
Before selection begins, however, the breeder should consider the heritability of the trait if it is quantitatively inherited. If heritability is high, an animal will transmit a large fraction of its superiority or inferiority to its offspring, but if heritability is low, the performance of an animal’s relatives as well as its own performance can be profitably used to increase the accuracy of selection. If heritability is very low, however, there is little use in seeking genetic improvement by ordinary methods of selection. Such is the case for most measures of fertility in domestic animals.
The objectives of plant breeding, as well as the techniques used, depend upon the kind of plant. For field, fruit, and vegetable crops, breeders develop varieties that resist disease, are adapted to the climate and soils of the region where they are grown, and produce high yields of good-quality seeds, forage, or fruits. For ornamentals, novelty of flower or foliage pattern often is the most important objective.
Many of the methods used in animal breeding are applicable to plant breeding; in addition, other methods that depend on the botanical characteristics of various species of plants may be employed. In general, pedigree breeding is used to improve the genotypes of self-fertilizing species, while backcrossing can be used with both self- and cross-fertilizing species. Mass selection and recurrent selection are used for “population building,” that is, for improving the average genetic value of a group of plants. Once a new variety has been developed, much more care is needed to retain its genetic purity if it is a cross-fertilizing species than if it is self-fertilizing.
A striking example of the value of modern breeding methods is provided by the discovery of the opaque-2 corn gene. By backcrossing this gene into local varieties, plant breeders can double or triple the amount of the amino acids, lysine and tryptophane, in corn protein. Since increased amounts of these two amino acids make corn protein more nearly equivalent to meat protein, this development promises to have a great impact on human nutrition in underdeveloped countries where diets are low in meat and high in cereal grains.