Although XX and XY are definitely the most common, differences in hormones and receptors in the body can lead to varying outward appearance (and inward feelings) about sex. For example, you can have XX chromosomes as well as congenital adrenal hyperplasia, where the body makes extra testosterone, leading to varying degrees of outward "male" development (if not managed with hormone medications). Similarly, you can have XY chromosomes, but have androgen insensitivity, where the body doesn't respon

An “Intersex” whale? Nearly all mammals use the XX/XY chromosomal system of sex determination, where females carry two X chromosomes and males carry XY. (In contrast, birds use the ZW system, in which males have two of the same sex chromosomes.) Last fall, a fascinating study documented the first case of an XXY chromosomal aneuploidy in a southern right whale. Differences in chromosome number are called aneuploidies, and this particular whale has what’s known as Klinefelter’s. News reports, howe