II.4 A single trait can be affected by different genes and a single gene can affect the occurrence and form of many traits.
Individual genes and, in fact, also individual factors in the external environment work together in various ways, interfere or replace one another in their effects in creating the final forms of the traits. Current molecular biological studies, for example, indicate that a major part of genetic information is redundant. If both copies of a certain gene (i.e. cistron) are artificially inactivated, it very frequently happens that the phenotype manifestation of the given mutation is very small or even negligible.
Experiments performed with baker’s yeast, for example, have shown that loss mutations in only 1100 genes (i.e. cistrons) of the total number of 6200 tested have lethal character. Inactivation of a further 291 genes has lethal character only if some other gene is simultaneously inactivated (Tong et al. 2001).
Fig. II.9 Epistasis and pleiotropy In epistasis, one trait is affected by a greater number of genes; in pleiotropy, one gene affects several different traits.
Genes or, to be more exact, their specific alleles can thus interact, i.e. mostly mutually augment or cancel their effects. In the case of an interaction within a single locus, we then speak of allele dominance in this context. If similar interactions occur between two different loci, then these are termed epistatic interactions. Of course, the actual interaction occurs physically as a rule, but not necessarily always, at the level of the products of the relevant genes and not at the level of the DNA.
Another phenomenon that greatly complicates the apparently simple relationship between a gene and trait is gene pleiotropy. A great many genes have pleiotropic effect: their presence, to be more exact the presence of a particular allele of the given gene, is manifested not in the form of a single trait, but rather in the form of a number of traits (Fig. II.9).