We will confine our attention to the simplest and most common: Although chromosomal status may influence development in a variety of ways, some of the most obvious effects are mediated through the gonads and sex endocrine system.
Gonadal dysgenesis or Turner's syndrome Although this is the commonest sex chromosome anomaly to arise, the large majority of fetuses so affected are aborted and consequently this karyotype occurs in less than 1 in live births.
Ovarian development starts normally, but presumably because of the absence of normal oocytes which require two X-chromosomes, see p. There is thus a marked deficiency of sex steroids, including an absence of the normal increase in oestrogens during the first post-natal year. This results in absence of breast development and secondary sexual characteristics, and amenorrhoea.
The genitalia are otherwise normally female. The most consistent feature is short stature, usually less than 58 inches 1. This has now been attributed to a genetic deficiency; with absence or rearrangement of part of the X chromosome that is relevant to linear growth Jaffe A variety of other somatic anomalies may occur, the best known being webbing of the neck see Fig. There are also a number of other health problems that may occur, including coarctation of the aorta and diabetes. Intelligence is usually normal, though spatial aptitude may be impaired.
Menstruation can be initiated and maintained with long-term oestrogen substitution therapy. This syndrome tells us little about the role of hormones in sexual development except that female pre-pubertal genitalia and gender identity develop in the absence of any ovarian steroid production.
Triple X This has a frequency of about 1 in live female births. Sexual development appears to be normal in most cases although puberty may be delayed and a proportion do not menstruate normally. At least some of these women are fertile producing normal children. They tend to be tall, and there may be learning disabilities.
Klinefelter's syndrome About 1 in newborn males has this karyotype. In some cases there may be three or even four X chromosomes. Development is invariably along male lines. The full picture of Klinefelter's syndrome includes small testes about 2—6 mL with tubular dysgenesis, hypogonadism, infertility, tall stature and gynaecomastia.
There may be intellectual impairment, more likely with each additional X chromosome, personality problems and abnormal sexual preferences. Considering the frequency of this condition at birth, the full syndrome is rare. This presumably means that the majority of such men must be relatively free from such stigmata. The endocrine pattern in this condition shows a somewhat low plasma T concentration though overlapping the normal range and raised gonadotrophins both LH and FSH.
Although the raised FSH is to be expected in view of the tubular dysgenesis, the high LH is probably not simply due to impaired Leydig cell function, as it is difficult to suppress the LH level to the normal range with exogenous androgens. These men usually have a relatively high oestrogen: T ratio which may account for their tendency to gynaecomastia.
The variability of phenotypes associated with XXY may well be explained by recent research on the X-linked androgen receptor gene carrying the CAGn repeat polymorphism, the length of which is inversely related to androgen action Zitzman et al The tall stature, which is by no means invariable, is not fully understood.
It is detectable throughout childhood before epiphyseal closure normally occurs, and is therefore not simply a result of delayed closure see Fig. Tubular degeneration in the testes proceeds throughout childhood and it is only in late childhood that the testis size and consistency is noticeably different from normal. Raboch et al found evidence of delayed development of heterosexual interest and socio-sexual behaviour in XXY males attending clinics.
A study of adolescent boys with XXY karyotypes identified at birth found them to have slightly lower intelligence than a group of normal matched controls Ratcliffe et al Most developed normally except for being behind their peer group in the onset of adolescent sexual interest, relatively impaired in peer group relationships and with more tender-minded personalities than the controls Bancroft et al One study found evidence of a positive feedback response in five adult males with Klinefelter's syndrome Barbarino et al This may reflect a relative lack of adult T rather than impaired early defeminization as, in contrast to the rat, suppression of positive feedback in the human male appears to depend on the prevailing hormonal milieu rather than on early defeminizing organization of the hypothalamus Gooren There is no evidence that XXY males are more likely to develop homosexual preferences.
A rarer and possibly related condition is that of the XX male. These men are similar in many ways to the XXY male, though not showing a tall stature to the same extent. There are no obvious sexual consequences except that testicular abnormalities with impaired spermatogenesis and tubular atrophy may occur Polani They are often fertile, fathering chromosomally normal children, though their fertility is apparently reduced by a low incidence of marriage.
The FSH may well reflect the tubular atrophy. The raised LH is more difficult to explain. Most studies have reported normal T levels, though one study found raised T compared with matched controls Schiavi et al These men show tall stature, similar to the XXY male, though their body shape is somewhat different.
They first attracted attention because of their apparently high incidence amongst patients of special hospitals i. It was suggested at one time that they had a tendency to high aggression, presumably a naive extrapolation from their extra Y chromosome.
In fact the XYY men found in the special hospital populations were characterized by offences against property rather than people. They showed inadequacy rather than aggression. It may be that they have an increased incidence of behavioural problems of various kinds, though the risk of an XYY male becoming convicted, although increased, is very small. As with XXY males, the majority live their lives without obvious problems or stigmata.
There is no evidence of an increased likelihood of homosexual development.