Figure 5.

Drawings taken from Darwin's The Different Forms of Flowers on Plants of the Same Species [5], illustrating the clear morphological distinction between the two floral types of Primula flowers. The 'thrum' (short-styled) form is determined by a dominant 'supergene' in the heterozygous state, the 'pin' (long-styled) form by the homozygous recessive. Thrum and pin forms are maintained in roughly equal numbers by the high fertility of thrum × pin crosses and near infertility of the two illegitimate thrum × thrum and pin × pin crosses. Although Darwin worked extensively on this beautiful qualitative trait, his interest was focused on the quantitative deficit of illegitimate progeny rather than the genetic control of the trait itself.