Polymeric and pleiotropic action, gene interaction problems

In polymeric action several non-allelic genes act in a similar way on the development of one trait; such genes mainly determine quantitative traits (height, mass, milk yield and fat content, egg production, sugar content). In cumulative polymery the trait strengthens with the number of dominant alleles; the Swedish scientist N. Nilsson-Ehle crossed red (A₁A₁A₂A₂) and white (a₁a₁a₂a₂) grained wheat varieties: F1 was pink, and F2 had red : light red : pink : light pink : white = 1 : 4 : 6 : 4 : 1. In non-cumulative polymery the trait appears if at least one dominant gene is present, giving 15:1 in F2. In pleiotropy one gene affects several traits: for example in Drosophila the gene for unpigmented (white) eyes also affects the colour of some internal organs and viability. A lethal gene causes death of the organism: in mice the yellow-coat gene (A) in the homozygous state (AA) kills the embryo, so Aa × Aa gives among the live-born 2 yellow : 1 black (not 3:1). Modern science also confirms that most traits (human height, crop yield) depend on many genes and on the environment.