Quantitative variation

Abstract

A characteristic feature of a trait showing quantitative variation is the great range in expression. Even in the absence of genetic variation, like in a clone, a pure line or an F₁ hybrid, there is a wide range of phenotypic values. For certain traits (quasi-continuous ones) one can easily distinguish different levels of expression, e.g. for the number of ears produced by different wheat plants; for other traits there is truly continuous variation of expression, e.g. for the grain yield of separate wheat plants or the length of their longest culm. In any case, the variation is such that it is impossible to distinguish, in a genetically heterogeneous population, plants according to their genotype simply on the basis of their phenotypic value. For so-called qualitative variation this is reasonably possible (however, dominance is a disturbing factor), and allows determination of the frequency of plants with a certain genotype. The expression of traits with such variation is mainly controlled by so-called major genes.