Unlike mammals, it’s not hormones that dictate a chicken’s sex. It’s a fundamental property of the cells themselves.
But this only became apparent when biologists investigated several odd chickens that were half male and half female, as if a line were drawn down the center of their bodies.
In mammals, there are two types of sex-determining chromosomes, X and Y. Each cell in an embryo has a pair of chromosomes, either XX or XY, but the cells are otherwise identical.
Then, early in development, in response to some environmental cue, a group of cells that will someday become ovaries or testes start to produce hormones that cause other cells to develop in male- or female-specific ways.
It’s the hormones that matter: Exposed to lots of testosterone and deprived of estrogen, cells with female chromosomes will form masculine tissues, and vice versa.
There are a few oddball species such as the duck-billed platypus which has a whopping 10 sex chromosomes, making males XYXYXYXYXY.
But the mammalian system was thought to represent a general rule among vertebrate species. And though birds have Z and W chromosomes rather than X and Y, and ZZ is male rather than female, they were thought to follow this rule, too.
Read complete article at: wired.com