Sex Chromosomes

In humans, the sex of an individual is determined by the type of sex chromosomes a person has. Most men have an X chromosome from their mother and a Y chromosome from their father. Most women receive an X chromosome from their mother and an X chromosome from their father. There are exceptions to this rule, such as an XO woman (Turner's syndrome), who receives only a single X from one of her parents and fails to receive a second X because of nondisjunction in her parent's sex chromosomes during meiosis. This might occur, for example, when in egg formation the mother's XX chromosomes fail to separate, resulting in one gamete receiving two X chromosomes and another gamete receiving no X chromosome. If a sperm fertilizes the egg that has no X chromosome, then the resulting zygote becomes an XO girl. Turner's syndrome affects about 1 out of every 3,000 female births. These females are characterized by having skin flaps on the back of their necks, and are sterile.

On the other hand, if a sperm containing a Y chromosome comes into contact with the XX combination in the mother's egg, then an XXY male will be produced. This is Klinefelter's syndrome, which is characterized by a sexually underdeveloped boy who has rudimentary testes and prostate glands, often no pubic or facial hair, long arms, and in some instances will develop breast tissue. About one of every 500 males born has Klinefelter's syndrome.

If a male gamete containing the Y chromosome fertilizes an egg containing no X chromosome, then that embryo will fail to develop because it is essential that every human must have at least one X chromosome. There appears to be just too much important genetic information on the X chromosome not to have one.

There are also special cases where, because of non-disjunction again, a woman can have 3 X chromosomes. These women are called triplo-X and can show tallness and have menstrual irregularities. Men who receive an X from their mother and a double Y from their fathers have an XYY combination and have a condition called Jacob's syndrome. When Patricia Jacobs first described this condition in 1965, she proposed the suggestion that the extra Y might cause increased aggression in these men that might land them in trouble with the law. In the early 70's, special counseling was given to these boys and their families to help them to avoid trouble. With continued research it was found that 96% of men with Jacob's syndrome are quite normal, although some may have acne, be a little taller, or may have speech and reading problems. By telling these boys and their families that they might become aggressive to the point of becoming criminals, they often fulfilled these expectations.

There are even special cases where a woman who is XXX and a man who is XYY can make all sorts of combinations in the production of their gametes. It is possible, for example, for an XXX woman to make an XX egg that could join with an XY sperm from an