The condition is usually characterized by the presence of an extra X chromosome. Klinefelter syndrome is estimated to occur in about one in 1,000 live born males and is found in all races, ethnic groups, and socioeconomic classes. Chromosomes are thin strands of genetic material found in cell nuclei. Chromosomes contain genetic information that is passed from parents to children and determine a person's unique, hereditary characteristics. In a fertilized egg, one chromosome from each pair of autosomes originates from the mother's egg, the other from the father's sperm.
Eggs normally contain a single X chromosome.
Sperm cells may contain either an X or Y chromosome. If a sperm bearing the X chromosome fertilizes the egg, the fetus will develop into a female. A sperm carrying the Y chromosome will produce a male fetus. Thus, when fertilization takes place, there results a total of 47 chromosomes in the fertilized egg instead of the normal number of 46. Whereas most males have one X and one Y chromosome, males with Klinefelter syndrome have an extra X chromosome that was present in the sperm or egg cell from which they later developed.