Humans have an additional pair of sex chromosomes for a total of 46 chromosomes. The sex chromosomes are referred to as X and Y, and their combination determines a person's sex. Typically, human females have two X chromosomes while males possess an XY pairing. This XY sex-determination system is found in most mammals as well as some reptiles and plants.
Whether a person has XX or XY chromosomes is determined when a sperm fertilizes an egg. Unlike the body's other cells, the cells in the egg and sperm — called gametes or sex cells — possess only one chromosome. Gametes are produced by
meiosis cell division, which results in the divided cells having half the number of chromosomes as the parent, or progenitor, cells. In the case of humans, this means that parent cells have two chromosomes and gametes have one.
All of the gametes in the mother's eggs possess X chromosomes. The father's sperm contains about half X and half Y chromosomes. The sperm are the variable factor in determining the sex of the baby. If the sperm carries an X chromosome, it will combine with the egg's X chromosome to form a female zygote. If the sperm carries a
Y chromosome, it will result in a male.
During fertilization, gametes from the sperm combine with gametes from the egg to form a zygote. The zygote contains two sets of 23 chromosomes, for the required 46. Most women are 46XX and most men are 46XY, according to the
World Health Organization.
There are some variations, though. Recent research has found that a person can have a variety of different combinations of sex chromosomes and genes, particularly those who identify as LGBT. For example, a certain X chromosome called Xq28 and a gene on chromosome 8 seem to be found in higher prevalence in men who are gay, according to a 2014 study in the journal
Psychological Medicine.
A few births out of a thousand of babies are born with a single sex chromosome (45X or 45Y) and are referred to as sex monosomies. Others are born with three or more sex chromosomes (47XXX, 47XYY or 47XXY, etc.) and are called sex polysomies. "In addition, some males are born 46XX due to the translocation of a tiny section of the sex determining region of the Y chromosome," said WHO. "Similarly some females are also born 46XY due to mutations in the Y chromosome. Clearly, there are not only females who are XX and males who are XY, but rather, there is a range of chromosome complements, hormone balances, and phenotypic variations that determine sex."
It is important to remember that sex and gender have two separate definitions and many cultures include more labels than simply "male" and "female" to identify others.