Sources of variation: scales

Abstract

Continuous variation is, as Johannsen showed, caused by partly heritable and partly non-heritable factors. By far the greater part of the heritable variation can be traced to differences in nuclear genes. These genic differences may give rise to variation of three kinds. The main and basic type of variation is that reflecting the effects of substituting one allele for another at any of the loci in the genetic system mediating the heritable variation of the character. In a diploid organism with two alleles at a locus, three genotypes can be distinguished in respect of that locus. Denoting the alleles as A and a, where A indicates the allele making for greater manifestation of the character and a the allele making for lesser manifestation, the three genotypes are aa, Aa and AA. Two substitutions of A for a are required to convert aa into AA. If the effects on the manifestation of the character of these two substitutions are equal, Aa being midway between the two homozygotes in its phenotype, and if the effect of substituting A for a is the same no matter which alleles are present at the other loci in the polygenic system, the variation associated with the A-a gene pair is entirely of this main or basic type and is completely described by the magnitude of effect of the substitution. Such variation may be said to be fixable in that its effects can be fixed as the difference between two true breeding lines.