January 21, 2023

Sex chromosome abnormalities

Sex chromosome abnormalities  are a group of genetic conditions that are caused by the loss or addition of one or both sex chromosomes.  They are the most commonly occurring chromosomal disorders in man, with an incidence of about 1 in 450 births and may affect the number or the structure of X and Y chromosomes. They are characterized by either gain or loss of entire sex chromosomes (aneuploidy), but abnormalities can also occur when a person is missing part of a sex chromosome. Chromosomal mosaicism, with a normal cell line and a second cell line with a numeric anomaly involving a sex chromosome, is not unusual.

Sex chromosome abnormalities are usually associated with a range of physical and developmental problems that are in general less severe than those caused by autosomal chromosomal abnormalities, such as trisomy 21 (Down syndrome. This is mainly due to the effect of random X chromosome inactivation, as well as the fact that Y chromosomes have a low gene content. Syndromes  caused by sex chromosome abnormalities are often hard to recognize at birth and may not be diagnosed until puberty, where there is a delay in normal sexual development. Prenatal diagnosis of these syndromes  is often incidental, in women undergoing testing for other reasons, such as advanced maternal age. In addition, the severity of the phenotype in patients with mosaicism is related to the percentage of abnormal cells among critical tissues

The best known sex chromosome syndromes in women are Turner (45,X) and 47,XXX syndromes and in males Klinefelter syndrome (47,XXY), and 47,XYY syndrome.