What is the genetic cause of trisomy X?
Trisomy X is a chromosome disorder. Typically, individuals have 23 pairs, or 46 total chromosomes. The chromosome pairs are numbered 1-22, with the final pair being known as the sex chromosomes. Generally, males have one X and one Y chromosome, whereas females typically have two X chromosomes. When a female has trisomy X, however, she has an extra X chromosome, or three total X chromosomes. This means that she has 47 total chromosomes.
Almost all cases of trisomy X occur spontaneously, most often due to an error in the production of an egg cell leading to it containing two copies, instead of the usual one, of the X chromosome. When an egg cell containing two X chromosomes is fertilized by a sperm cell containing one X chromosome, the baby ends up with three copies of that chromosome. This error in chromosome packaging is known as "nondisjunction" and does happen more frequently as women age. Therefore, the chance of having a baby with trisomy X does increase with a woman's age.
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How does an extra chromosome cause the features of trisomy X?
The issues associated with trisomy X are caused by the extra X chromosome, which is present from the moment the egg and sperm cell come together. In most females, there are two X chromosomes and one of them is inactivated or "turned off" in each cell. This is because most genes on the X chromosome need only one copy to be turned on and explains why the genes on the X chromosome function normally in both females, who typically have two copies of each of these genes, and males, who typically have only one X chromosome and therefore one copy of each of these genes. In females with trisomy X, however, if one copy of the X chromosome is inactivated, that leaves two copies turned on. This overexpression of genes on the X chromosome is thought to be the cause of the symptoms of trisomy X; however, the exact mechanism of this is not well understood.
Does anything make trisomy X worse?
There is nothing known to make the symptoms of trisomy X worse.