Humans are born with 46 chromosomes in 23 pairs. Most women are 46XX and most men are 46XY. Research suggests, however, that in a few births per thousand some individuals will be born with a single sex chromosome 45X or 45Y sex monosomies and some with three or more sex chromosomes 47XXX, 47XYY or 47XXY, etc. In addition, some males are born 46XX due to the translocation of a tiny section of the sex determining region of the Y chromosome.
Abnormalities of the sex chromosomes
WHO | Gender and Genetics
About 1 in male and 1 in female live births demonstrate some form of sex chromosome abnormality, although the symptoms of these conditions are generally much less severe than are those associated with autosomal abnormalities. Turner syndrome is a condition of females who, in the classic form, carry only a single X chromosome 45,X. Turner syndrome is characterized by a collection of symptoms, including short stature, webbed neck, and incomplete or absent development of secondary sex characteristics, leading to infertility. Although Turner syndrome is seen in about 1 in 2, to 1 in 5, female live births, the 45,X karyotype accounts for 10 to 20 percent of the chromosomal abnormalities seen in spontaneously aborted fetuses, demonstrating that almost all 45,X conceptions are lost to miscarriage. Indeed, the majority of liveborn females with Turner syndrome are diagnosed as mosaics , meaning that some proportion of their cells are 45,X while the rest are either 46,XX or 46,XY. The degree of clinical severity generally correlates inversely with the degree of mosaicism, so that females with a higher proportion of normal cells will tend to have a milder clinical outcome.
Gender and Genetics
The term Klinefelter syndrome KS describes a group of chromosomal disorder in which there is at least one extra X chromosome to a normal male karyotype, 46,XY. XXY aneuploidy is the most common disorder of sex chromosomes in humans, with prevalence of one in males. Other sex chromosomal aneuploidies have also been described, although they are much less frequent, with 48,XXYY and 48,XXXY being present in 1 per 17, to 1 per 50, male births.
Disorders of the sex chromosome like disorders of the autosomes can be either numerical or structural, and can be present in all cells or in a mosaic form. Clinical indications that should raise suspicions of a sex chromosome abnormality are:. These disorders are considered very common with incidence of about —