Mammals have evolved to be male (XY chromosomes) or female (XX chromosomes.) However, as is the case for every form of cell division, it is always possible for a chromosome to be broken, duplicated, or sorted incorrectly during meiosis. As a result, a small percent of mammals (including, but not limited to, people) may be born with one of the following conditions. Note that this is just a partial list. There are many more possibilities.
The result of these variations are such that many are not 100% male or female. There will be sometimes significant variation in terms of appearance, reproductive ability, interior sex organ structure, exterior sex organ structure, or neuroanatomy (structure of certain parts of the brain, which are only visible with techniques such as MRI scans.) Many people born with such conditions are intersex or transgendered.
Here are six of the more common sex chromosome variations.
When one is missing, or has an extra, sex chromosome, what are the results?
There are a number of unusual forms of sexual differentiation, which are caused by certain hormones or drugs.
What factors lead to a person’s gender identity?