Well, I would think that having a 4-sex-system would lead to all possible combinations of those. Let's say there are A, B, C and D individuals. You would have a

Well, I would think that having a 4-sex-system would lead to all possible combinations of those. Let's say there are A, B, C and D individuals. You would have a breed from A x B, A x C, A x D, B x C, B x D and C x D. But what I got is that there males and females. But some of them have a different chromosome, to that it's transmitted for their little cute birdies. Let's call it n and m versions, and keep the males (Y) and females (X) So you have Ym, Yn, Xm and Xn. You just have Ym x Xm or Ym x Xn. OR Yn x Xm or Yn x Xn. You don't have Ym x Yn. Nor Xm x Xn. You just have 4 possible combinations, not the whole 6 combinations if there really were 4 different sex. Perhaps I'm missing something from the article, from the paper, or from biology, but since those special mutation from the chromosome 2 doesn't generate infertile offspring, I simply understand it as "birds with a white hair prefer those with red hair, instead of opposite-sex with the same hair color, generating a offspring that will have white or red hair with the same probability."