| MadSci Network: Cell Biology |
Frederic,
A good question that relates to the very active field of research trying to find out exactly how sex determination happens.
The short answer is that it is not possible to have a living human being without an X chromosome. That is because the X chromosome carries genes that are necessary for the life of both men and women, in addition genes involved in choosing sex. It is the Y chromosome that is dispensible, as woman (XX) demonstrate.
If we assume, for a moment, that the X chromosome was just for sex determination, then the best guess is that YY people would be male. That is because the Y chromosome contains genes that make you male: if you have it you are male (XY usually), if you don't you are female (XX). In other words, the choice to be female is the default choice, while the choice to be male is determined by genes on the Y chromosome.
You can learn more about how chromosomes determine sex by looking at some of the natural abnormalities that occur, similar to the YY abnormality you suggested. For example, people that are XXY are male. This shows that one Y is enough to make a person male, even when there are two X chromosomes. However, XXY people have a disease called Klinefelter's syndrome, part of which is underdevelopment of the testes, implying that the number of X chromosomes also influences becoming male. Another abnormality, much rarer than Klinefelter's, occurs when people have one X and two Y chromosomes. These XYY people are also male, large in stature, and many of them are psychopathic. That of course shows that too much Y isn't good for you in any case!