The phenotype of a plant is determined by its genetic composition, the environment in which the plant is grown, and the interaction of genotype with environment. The challenge plant breeders face is to identify and select those plants that have genotypes conferring desirable phenotypes, rather than plants with favorable phenotypes due to environmental effects. Narrow sense heritability is a measure of the ratio of additive genetic variation to phenotypic variation in a given population for a given trait. As a rule, traits with greater heritability can be modified more easily by selection and breeding than traits with lower heritability. Narrow sense heritability for a number of traits in tall fescue populations has been reported (Table 19-2).
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Additive genetic variation generally is of most interest to tall fescue breeders because typically it makes a proportionally greater contribution to genetic variation than other types of gene action and is more easily captured by the commonly used methods of developing tall fescue cultivars. Additive genetic variation, as estimated by general combining ability, has been found to be prominent in tall fescue for traits such as forage yield components (Volenec et al., 1984), seed yield components (Nguyen and Sleper, 1985), in vitro dry matter digestibility (IVDMD), detergent fiber parameters (Nguyen et al., 1982), and ozone resistance (Johnston et al., 1983). Other types of genetic variation, including dominance and epistasis, appear to be of lesser importance in tall fescue, as indicated by lower levels of specific combining ability in these same studies.
Traits can be classified as either qualitative, whereby variation occurs in discrete classes and often is controlled by one or a few genes, or quantitative, where variation is continuous and often is controlled by several genes, each having a minor effect. Resistance to stem rust (caused by Puccinia graminis Pers.:Pers. subsp. graminicola Z. Urban) is an example of what is probably a qualitative trait in tall fescue (Barker et al., 2003), whereas quantitative traits would include seed yield, forage yield, persistence, and digestibility. Quantitative traits can be influenced greatly by environment and thus have lower heritabilities than qualitative traits. Plant breeders generally are interested mostly in improving quantitative traits of tall fescue.