I thought that the X and Y chromosomes determined gender in animals, but I see no mention of them in your article. Does this mean that on a genetic basis, males and females in these organisms are identical?

Neil H. Murphy
Walnut Creek, Calif.

In a sense, yes, says Philip Hastings of the Scripps Oceanographic Institute in La Jolla, Calif. Sex-changing fish don’t rely on chromosomes to determine sex, so there’s no special DNA difference between males and females. However, other fish species, such as swordtails and guppies, do depend on chromosomes to determine sex .—S. Milius
Use up and down arrow keys to explore.Use right arrow key to move into the list.Use left arrow key to move back to the parent list.Use tab key to enter the current list item.Use escape to exit the menu.Use the Shift key with the Tab key to tab back to the search input.