Polygenic sex determination

What is sex determination? Sex determination is the mechanism by which sexual organisms direct gonad development towards distinct but reproductively compatible outcomes. Molecular signaling cascades in the developing gonad provide instructions to the tissue to develop as male or female (or in some cases hermaphrodite), and these signals can be initiated in different ways. Sex can be determined by environmental cues (such as temperature), or genetically. Genetic sex determination occurs when an inherited difference in genes or chromosomes initiates the sex determination signal, acting as a 'master switch' for male or female development. Perhaps the most widely known genetic sex determination system is the male heterogametic XX/XY system found in most mammals, where the Sry gene on the Y chromosome determines male sex. Thus, XY individuals develop as males, and XX individuals develop as females. Many other organisms, including birds, have a ZZ/ZW chromosomal system, where heterogametic ZW individuals develop as female, and homogametic ZZ individuals develop as male (Figure 1).