Figure 2.

Monogenic, digenic and polygenic male determination. The first three panels denote monogenic sex determination, since a single gene A on a sex chromosome (blue bar) directs male determination, either by (a) its presence/absence, (b) through multiple alleles) or (c) by initiating a biochemical pathway. Panel (d) represents a tetraploid with two copies of gene A on two copies of the sex chromosome; this would produce a digenic system if both a copies were active (e.g. via A dosage), or would revert to a monogenic system if one copy A’ degenerates or changes its function. Panel (e) illustrates a digenic system in transition in which an original XY system is being taken over by a novel WZ system. Panel (f) illustrates a truly polygenic system in which four genes at distinct loci on three chromosomes contribute additively toward male determination. Blue bars, male promoting chromosome systems, green, female promoting chromosomes.