Chromosomes Chromosomes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic... read more 
are structures within cells that contain DNA DNA Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic... read more and many genes. Genes Genes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic... read more are segments of deoxyribonucleic acid (DNA) and contain the code for a specific protein that functions in one or more types of cells in the body. Genes contain instructions that determine how the body is supposed to look and function. (See Genes and Chromosomes Genes and Chromosomes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic... read more for a discussion about genetics.)
Except for certain cells (for example, sperm and egg cells or red blood cells), every human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes. There are 22 pairs of chromosomes that are not sex chromosomes (called nonsex chromosomes, numbered chromosomes, or autosomal chromosomes) and one pair of sex chromosomes.
The sex chromosomes Sex chromosomes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic... read more determine whether a fetus becomes male or female. A pair of X and Y chromosomes (XY) results in a male, and a pair of X and X chromosomes (XX) results in a female.
Sex chromosome abnormalities occur when a person is missing a whole sex chromosome (called monosomy) or has an extra sex chromosome (called a duplication). Abnormalities can also occur when a person is missing part of a sex chromosome (called a deletion).
Sex chromosome abnormalities are common and cause syndromes that are associated with a range of physical and developmental problems. Many of these syndromes are not noticed while the mother is pregnant but may be discovered if prenatal testing Prenatal Diagnostic Testing Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more is done for other reasons, such as older age in the mother. The abnormalities are often hard to recognize at birth and may not be diagnosed until puberty.
Syndromes that are caused by a sex chromosome abnormality are less severe than those caused by a nonsex chromosome abnormality. For example, girls who have an extra sex chromosome (an extra X) often appear normal physically and mentally and are fertile. In contrast, children who have an extra nonsex chromosome typically have a severe disorder, such as trisomy 13 Trisomy 13 Trisomy 13 is a chromosomal disorder caused by an extra chromosome 13 that results in severe intellectual disability and physical abnormalities. Trisomy 13 is caused by an extra chromosome 13... read more , which results from a person having an extra chromosome 13.
Similarly, girls who are missing a sex chromosome have a specific syndrome (Turner syndrome Turner Syndrome Turner syndrome is a sex chromosome abnormality in which girls are born with one of their two X chromosomes partially or completely missing. Turner syndrome is caused by the deletion of part... read more 
), whereas fetuses who are missing a nonsex chromosome do not survive.
(See also Overview of Chromosome and Gene Disorders Overview of Chromosome and Gene Disorders Chromosomes are structures within cells that contain a person's genes. Genes are segments of deoxyribonucleic acid ( DNA) and contain the code for a specific protein that functions in one or... read more .)
