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.
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.
The existence of interactions greatly complicates both the search for the locus at which the gene determining a certain trait is located and also the delimitation of the particular gene. Basically, it even complicates the very concept of a gene and especially the molecular biological definition of a gene as a cistron. If the interaction amongst several genes, and not a particular gene, is responsible for the formation of a particular trait, then there is no point in searching for the locus at which the particular gene is located. However, interactions reduce the effectiveness of the action of natural selection in evolution. If several genes located at various places on the genome participate in the formation of a particular trait, then its heritability is substantially reduced (see also Section II.7). The trait in the original form in which it occurred in the parents will develop only in those progeny that have the same allele at all the participating loci as their parents. However, in a polymorphous population, recombination and segregation of chromosomes leads to mixing of the genes of the two parents so that the probability that any of the progeny would inherit exactly the same combination of alleles as one of the parents would be very low. If the particular combination of alleles and thus the particular trait is transferred from the parents to the progeny, it is very probable that the particular allele will fall apart in one of the subsequent generations. See also Frozen plasticity theory.