Humans have 22 chromosome pairs and two sex chromosomes. Females have two X chromosomes; males have an X chromosome and a Y chromosome.
Humans have 22 chromosome pairs and two sex chromosomes. Females have two X chromosomes; males have an X chromosome and a Y chromosome.
Credit: U.S. National Library of Medicine

Chromosomes are thread-like molecules that carry hereditary information for everything from height to eye color. They are made of protein and one molecule of DNA, which contains an organism's genetic instructions, passed down from parents. In humans, animals, and plants, most chromosomes are arranged in pairs within the nucleus of a cell. Humans have 22 of these chromosome pairs, called autosomes.

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. 

While the chromosomes for other parts of the body are the same size and shape — forming an identical pairing — the X and Y chromosomes have different structures.

The X chromosome is significantly longer than the Y chromosome and contains hundreds more genes. Because the additional genes in the X chromosome have no counterpart in the Y chromosome, the X genes are dominant. This means that almost any gene on the X, even if it is recessive in the female, will be expressed in males. These are referred to as X-linked genes. Genes found only on the Y chromosome are referred to as Y-linked genes, and expressed only in males. Genes on either sex chromosome can be called sex-linked genes.

There are approximately 1,098 X-linked genes, though most of them are not for female anatomical characteristics. In fact, many are linked to disorders such as hemophilia, Duchenne muscular dystrophy, fragile-X syndrome and several others. They are responsible for red-green color blindness, considered the most common genetic disorder and found most often in males. The non-sex feature X-linked genes are also responsible for male pattern baldness.

In contrast to the large X chromosome, the Y chromosome contains only 26 genes. Sixteen of these genes are responsible for cell maintenance. Nine are involved in sperm production, and if some are missing or defective, low sperm counts or infertility may occur. One gene, called the SRY gene, is responsible for male sexual traits. The SRY gene triggers the activation and regulation of another gene, found on a non-sex chromosome, called the Sox9. The Sox9 triggers the development of non-sexed gonads into testes instead of ovaries. 

Abnormalities in the sex chromosome combination can result in a variety of gender-specific conditions that are rarely lethal.

Female abnormalities result in Turner syndrome or Trisomy X, according to the U.S. National Library of Medicine. Turner syndrome occurs when females have only one X chromosome instead of two. Symptoms include failure of the sex organs to normally mature, which may lead to infertility, small breasts and no menstruation; short stature; a wide, shield-shaped chest; and a wide, webbed neck.

Trisomy X syndrome is caused by three X chromosomes instead of two. Symptoms include tall stature, speech delays, premature ovarian failure or ovarian abnormalities, and weak muscle tone — although many girls and women exhibit no symptoms.

Males can be affected by Klinefelter syndrome. Symptoms include breast development, abnormal proportions such as large hips, tall height, infertility, and small testicles.

Additional resources