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. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or more types of cells in the body ( 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). Genes contain instructions that determine how the body is supposed to look and function. 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 Chromosome abnormalities 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 occur when a person is missing a whole sex chromosome (called monosomy) or has an extra sex chromosome (one extra is trisomy). 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 Testing for chromosome and gene abnormalities Chromosomes are structures within cells that contain a person's genes. A gene is a segment of deoxyribonucleic acid ( DNA) and contains the code for a specific protein that functions in one... 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 extra numbered (1 to 22 ) chromosomes typically have severe abnormalities such as Down syndrome Down Syndrome (Trisomy 21) Down syndrome is a chromosome disorder caused by an extra chromosome 21 that results in intellectual disability and physical abnormalities. Down syndrome is caused by an extra chromosome 21... read more 
, which commonly results from a person having an extra chromosome 21. An extra chromosome 1 can be fatal to a fetus. 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.
