BIOLOGY Volume 3 - A Guide to General Biology - 2004

25. APPLIED GENETICS

25.7. Human Genetics

25.7.8. Turner Syndrome

The syndrome was first described in 1938 by the American endocrinologist Turner. Women with Turner syndrome typically present with the following features:

1) short stature (approximately 150 cm);

2) webbed neck;

3) thick fingers with deeply set, abnormally convex Nails;

4) low hairline at the nape of the neck;

5) streak Ovaries (consisting only of Connective Tissue);

6) hypoplastic Uterus.

In 1959, it was established that Turner syndrome is caused by the absence of an X chromosome. Affected individuals have 45 Chromosomes instead of 46, with an XO sex chromosome Complement instead of XX. This genotype is an example of monosomy. As illustrated in Fig. 25.29, Turner syndrome can arise As a result of nondisjunction during Meiosis, representing a counterpart to Klinefelter syndrome. Theoretically, individuals with Klinefelter and Turner syndromes should be born in equal numbers. In reality, however, Turner syndrome occurs much less frequently—approximately 1 in 2,500 live births—compared to 1 in 500 live births for Klinefelter syndrome. This discrepancy is due to the significantly higher rate of spontaneous Miscarriage among fetuses with Turner syndrome. It is estimated that only 2–3% of embryos with Turner syndrome survive to term. Notably, Turner syndrome appears to be a major cause of early miscarriages, with 20% of such embryos having an XO genotype.

Treatment

Beginning at Puberty, Female Sex Hormones are administered to stimulate breast development. While Infertility cannot be overcome in these cases, hormone therapy improves physical appearance and enhances the patient's psychological well-being.

Other Sex Chromosome Combinations

Various other sex chromosome abnormalities have been described. Individuals possessing a Y chromosome are invariably male due to the presence on this chromosome of the Gene (SRY) responsible for male Sex Determination. Several phenotypic males with an XX genotype have been identified (1 in 20,000 live births); however, it has been shown that in such men, a segment of the Y chromosome carrying the SRY gene has been translocated onto one of the X chromosomes. Phenotypically, they resemble patients with Klinefelter syndrome. Males with XXXY and XXXXY sex chromosome complements also occur and are characterized by mental retardation. Females with four X chromosomes experience severe intellectual disability, despite a normal physical appearance. In 1961, the first XYY male was identified. Although phenotypically normal, further research is required to determine whether such patients exhibit any subtle physiological or psychological abnormalities. XXYY males have also been discovered.