Numerical abnormalities of the sex chromosomes are fairly common and their effects are much less severe than those caused by autosomal abnormalities. Sex chromosome
abnormalites are often detected coincidentally at amniocentesis or during investigation for infertility. Many cases are thought to cause no associated problems and to remain undiagnosed. The risk of recurrence after the birth of an affected child is very low. When more than one additional sex chromosome is present learning disability or physical abnormality is more likely.
Turner syndrome is caused by the loss of one X chromosome (usually paternal) in fetal cells, producing a female conceptus with 45 chromosomes. This results in early spontaneous loss of the fetus in over 95% of cases. Severely affected fetuses who
survive to the second trimester can be detected by ultrasonography, which shows cystic hygroma, chylothorax, asictes and hydrops. Fetal mortality is very high in these cases. The incidence of Turner syndrome in liveborn female infants is 1 in 2500. Phenotypic abnormalities vary considerably but are usually mild. In some infants the only detectable abnormality is lymphoedema of the hands and feet. The most consistent features of the syndrome are short stature andinfertility from streak gonads, but neck webbing, broad chest, cubitus valgus, coarctation of the aorta, renal anomalies and
visual problems may also occur. Intelligence is usually within the normal range, but a few girls have educational or behavioural problems. Associations with autoimmune
thyroiditis, hypertension, obesity and non-insulin dependent diabetes have been reported. Growth can be stimulated with androgens or growth hormone, and oestrogen replacement treatment is necessary for pubertal development. A proportion of girls with Turner syndrome have a mosaic 46XX/45X karyotype and some of these have normal gonadal development and are fertile, although they have an increased risk of early miscarriage and of premature ovarian failure. Other X chromosomal abnormalities including deletions or rearrangements can also result in Turner syndrome.
The XXY karyotype of Klinefelter syndrome occurs with an incidence of 1 in 600 live born males. It arises by nondisjunction and the additional X chromosome is equally
likely to be maternally or paternally derived. There is no increased early pregnancy loss associated with this karyotype. Many cases are never diagnosed. The primary feature of the syndrome is hypogonadism. Pubertal development usually starts spontaneously, but testicular size decreases from mid-puberty and hypogonadism develops. Testosterone replacement is usually required and affected males are infertile. Poor facial hair growth is an almost constant finding. Tall stature is usual and gynaecomastia may occur. The risk of cancer of the breast is increased compared to XY males. Intelligence is generally within the normal range but may be 10–15 points lower than siblings. Educational difficulties are fairly common and behavioral disturbances are likely to be associated with exposure to stressful environments. Shyness, immaturity and frustration tend to improve with testosterone replacement therapy.
The XYY syndrome occurs in about 1 per 1000 live born male infants, due to nondisjunction at paternal meiosis II. Fetal loss rate is very low. The majority of males with this karyotype have no evidence of clinical abnormality and remain undiagnosed.
Accelerated growth in early childhood is common, leading to tall stature, but there are no other physical manifestations of the condition apart from the occasional reports of severe acne. Intelligence is usually within the normal range but may be about 10 points lower than in siblings and learning difficulties may require additional input at school. Behavioral problems can include hyperactivity, distractability and impulsiveness. Although initially found to be more prevalent among inmates of high security institutions, the syndrome is much less strongly associated with aggressive behavior than previously thought although there is an increase in the risk of social maladjustment.
Triple X syndrome
The triple X syndrome occurs with an incidence of 1 in 1200 liveborn female infants and is often a coincidental finding. The additional chromosome usually arises by a nondisjunction error in maternal meiosis I. Apart from being taller than average, affected girls are physically normal. Educational problems are encountered more often in this group than in the other types of sex chromosome abnormalities. Mild delay with early motor and language development is fairly common and deficits in both receptive and expressive language persist into adolescence and adulthood. Mean intelligence quotient is often about 20 points lower than that in siblings and many girls require remedial teaching although the majority attend mainstream schools. The incidence of mild psychological disturbances may be increased. Occasional menstrual problems are reported, but most triple X females are fertile and have normal offspring. Early menopause from premature ovarian failure may occur.