II.7 Trait heritability describes the share of the genetically determined variability of the trait in its total variability.
Individual traits vary in their heritability. The trait heritability of qualitative traits expresses the probability that the traits will be transferred in unaltered form to the next generation while, for quantitative traits, this corresponds to the degree to which they are transferred from one generation to the next. The heritability of a trait is, however, defined in genetics as the fraction of genetically determined variability in the given trait in the total variability of this trait, i.e. also the environmentally determined variability of this trait. Some components of genetically determined variability are inherited from one generation to the next, while others are not. Consequently, attention is concentrated especially on heritability in the narrow sense of the word, i.e. the component of variability of a given trait determined by genes whose effects can be simply added, i.e. genes with additive effect. Other components of heritability include environmentally determined variability, variability determined by interactions between alleles in a single locus (the dominance component) and variability determined by interactions between alleles in various loci (the epistasis component) It is, of course, possible to also define other components of the total variability, with contributions, e.g., from interactions between the environment and dominance, interactions between dominance and epistasis, etc. The evolution of a certain trait through selection is fundamentally affected by the additive genetically determined variability; the other components of the variability mostly reduce the effectiveness of selection.
Morphological traits have greater heritability in the narrower sense of the word (hereinafter “heritability”), while behavioural traits have lower heritability and traits immediately affecting the biodemographic parameters (life history characteristics) of the population mostly have the lowest heritability; the latter include, e.g., the life expectancy, speed of attaining maturity, number of progeny, etc. (Mousseau & Roff 1987; Hoffmann 2000; Bouchard & Loehlin 2001). In general, it can be stated that traits determined by a small number of genes, in the ideal case by a single gene, tend to have greater heritability and, on the other hand, traits whose presence or absence has a great effect on the biologically fitness of their carriers, have low heritability. The low heritability of polygenically determined traits follows from the mechanism of processing genetic information (see above). The low heritability of traits with a high impact on biological fitness follows from the fact that these traits very readily succumb to selection so that, if a new allele appears in the population, affecting the particular trait, it is very rapidly fixed or, on the other hand, eliminated from the population. A large fraction of the variability in the given trait surviving in the population is thus of nongenetic nature or is the result of polygenetic traits exhibiting low heritability.