Sex chromosome aneuploidies
When conceived, the embryo receives an X chromosome from the mother and the X or Y chromosome from the father. That means that the father (or his genes, to be exact) determines the child's gender at conception. If the father contributes the X chromosome, it will be a girl, and if he contributes the Y, a boy. That is, of course, the ideal scenario, but it does not always happen. In rare cases, sex chromosomes defects, also called aneuploidies of sex chromosomes, can occur. They represent approximately 12% of all chromosomal abnormalities.
They can occur in both men and women, but not all in both sexes. Sexual chromosome defects cause various clinical signs from physical characteristics, developmental and learning disorders to infertility.
Sex chromosome aneuplodies among men
Klinefelter syndrome (XXY)
Men affected by Klinefelter syndrome have an additional X chromosome in each body cell.
The main symptom is small testicles, not producing enough testosterone before birth and during puberty. Testosterone deficiency, however, inhibits the normal development of male characteristics. Men affected by Klinefelter syndrome have a less hairy face and body. They may also develop breast tissue, increasing their risk of breast cancer; but this risk is still lower than in women. In many cases, the syndrome leads to infertility, and it can lead to weaker muscles, higher stature, and inadequate libido. Usually, the syndrome does not affect intelligence, but reading and speaking problems may occur in some cases. There is no cure for the syndrome, but various physical, speech, and hormonal or testosterone therapies can help. From time to time, the adjustment of learning methods is also necessary.
Klinefelter syndrome occurs randomly and is not hereditary. It is present in 1 to 2 live-born boys per 1,000 births. This makes it the most common among all syndromes caused by sex chromosome abnormalities. It is also more common than Down syndrome. The reason is most likely the fact that life with this syndrome can be reasonably normal. As adults, most men with the syndrome lead lives similar to those without it.
Jacob's syndrome (XYY)
XXY syndrome, also called Jacob's syndrome, affects men and is caused by an additional Y chromosome.
Affected individuals are usually very tall, and many have severe acne problems at a young age. Some also experience learning difficulties, behavioural issues, and impulsivity. However, the presence of an additional Y chromosome does not lead to infertility and a decrease in intelligence, so people with this syndrome can live a normal life.
Sex chromosome aneuplodies among women
Turner syndrome (X)
Turner syndrome is caused by a total or partial absence of sex chromosome X in women.
The syndrome has a wide range of symptoms: the two most common are low stature and underdeveloped ovaries. The latter results in the absence of period, infertility, and less developed breasts. Women with only one X chromosome may also experience heart defects, diabetes, and thyroid problems. However, most do not have problems with mental development and are normally intelligent.
Some women may have visual, hearing, or spatial performance problems. Turner syndrome is often incompatible with life, and the affected fetus does not survive until birth. If born, there is no cure for Turner syndrome, but therapies can help alleviate symptoms. Growth hormone injections in childhood can help with more normal growth, and estrogenic treatments can help develop breast and ovarian tissue.
Turner syndrome is not usually hereditary. It occurs during the formation of the parent's reproductive cells or during cell division at an early development stage. Turner syndrome appears in about one girl among 2,000 to 5,000 births. Chromosome abnormality is sometimes present in only some of the cells – in this case, the abnormality is called Turner syndrome with mosaicism. In such cases, a person usually has fewer or no symptoms at all.
World-famous actress Linda Hunt is one of the most successful and recognisable actresses despite being diagnosed with Turner Syndrome.
Triple X syndrome (XXX)
Triple X syndrome occurs when a woman has an additional X chromosome in each cell.
Some women have very mild symptoms or none at all, so the syndrome might even remain undiagnosed. Women with this syndrome are often taller than average and may have difficulty learning. Delayed speech development and underdeveloped motor skills (such as sitting and walking) can also occur. They can have weak muscle tone (hypotonia), behavioural disorders and emotions difficulties. About 10 per cent of those affected with Triple X syndrome experience kidney problems. Some of these problems can be alleviated or eliminated with speech and physical therapies. Women who have an additional X chromosome present have no fertility problems and usually have normally developed intelligence.
The syndrome appears in about one in 1,000 live births. It is estimated that as many as 90 per cents of girls with the syndrome are left undiagnosed as they have zero or very few symptoms, and can live a full and normal life.
Although sex chromosome defects tend to have milder consequences than other chromosomal abnormalities, it is still crucial to know about them. This way, parents can properly prepare for the birth of a child with the syndrome. The NIPT test can predict the presence of sexual chromosome defects from the 10th week of pregnancy. If the analysis detects such aneuploidies, the parents have enough time to learn about the health consequences and provide the child with appropriate post-birth assistance and/or therapies.