Figure 2:

A simplified mechanistic model for a PSD system based on the involvement of four autosomal genes. In this theoretical PSD system, protein products of four genomic loci determine sex. Two of the protein products perform a function that pushes the gonad toward femaleness D and E loci, while the remaining two are proteins with pro-male function A and B loci. For simplicity, it is assumed that (i) for every locus there is a strong (larger shape and upper case letter) and weak (smaller shape and lower case letter) effect allele; (ii) the effect of the four strong alleles are equal and the same is true for the four weak alleles at a lower level and (iii) the four products do not exert any direct effect on the functions of each other. If we assign a binary code to the alleles (strong—1 and weak—0), then the outcome could be predicted by simply comparing the sum of male and female alleles (assuming that in case of a tie the individual would continue to develop into a female). In this system, a weak male that has two strong male alleles and one strong female allele (A) could produce sperm cells of different sexual genotypes [see (B) and (C) for examples], among them some that would have excess of pro-female alleles [see (C)]. Similarly, a weak female with one strong male and two strong female alleles (D) could also produce oocytes with excess strong female alleles (E) or more strong male alleles (F). Symbols with different shapes label genes located on four different autosomes. Ratios of strong male to strong female alleles are indicated in brackets.