Trends Genet 27 : — Namespaces Article Talk. These data suggest that there are constraints on the types of genes that two kinds of sex chromosomes in determining in Queanbeyan be co-opted as master sex determination genes .
Even though the outcome of sex determination—whether an individual produces relatively few large ova or many small sperm—is strongly conserved, a bewildering number of underlying mechanisms can trigger development as either a male or female .
Turnover of sex chromosomes can also be triggered by the degeneration of the Y and W chromosome, which commonly follows the cessation of recombination and will result in the replacement of a low-fitness Y or W chromosome with a nondegenerate one . Further extensions of the MAT region, possibly involving additional sex-antagonistic genes, led to the U and V chromosomes characterizing male and female gametophytes, as found, e.
In this system, most females have two of the same kind of sex chromosome XXwhile most males have two distinct sex chromosomes XY. By contrast, in birds, Drosophilaand nematodes  — sexual differentiation is a cell-autonomous process, although secreted signaling molecules can play a role in generating sexual dimorphism in somatic tissues.
Figure 2 highlights the possible pathways for the evolution of separate sexes from a hermaphrodite ancestor and illustrates their relation to sex chromosome evolution. In some species of reptiles, including alligators , some turtles , and the tuatara , sex is determined by the temperature at which the egg is incubated during a temperature-sensitive period.
In this variant of the XY system, females have two copies of the sex chromosome XX but males have only one X0. Ashley; D. Simultaneous hermaphroditism: male and female sexual organs coexist in one individual e.