Human genetics : prepared by the Standing Medical Advisory Committee.

  • Great Britain. Standing Medical Advisory Committee.
Date:
1972
Catalogue details

Licence: Open Government Licence

Credit: Human genetics : prepared by the Standing Medical Advisory Committee. Source: Wellcome Collection.

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    Chromosome Abnormalities 20. In the past decade there have been major advances in the field of human cytogenetics. In 1956 it was shown that the normal human chromo- some number was 46 not 48 as had been thought for the preceding thirty years. The application of new techniques to stop cell division at a stage when the chromosomes are clearly visible in the cells of bone marrow, solid tissues and peripheral blood etc has paved the way for recent dis- coveries which have shown that abnormal chromosomal patterns are associated with specific clinical abnormalities. It is now known that about 1 per cent of all liveborn children and about a quarter of fetuses spon- taneously aborted in the first trimester have chromosome abnormalities. As about 15 per cent of all known pregnancies are believed to end spon- taneously in first trimester abortion it would seem that about 4 per cent of all zygotes which last long enough to be recognised have chromosome abnormalities. 21. Chromosome abnormalities may be divided into those where the abnormality is of number, usually an extra chromosome, and those of structure where one or more chromosome breaks have determined loss of, or rearrangement of, chromosomal material. The best known of the con- ditions associated with abnormal chromosome number is Down’s Syn- drome (Mongolism) which has an incidence of about 1 in 600 livebirths and is responsible for about a quarter of all severely mentally handicapped children of school age. The abnormality usually consists of an extra chromosome identical to the members of the normal pair number 21, so that they have three No 21 chromosomes (trisomy 21) and 47 chromosomes altogether. Here the frequency is closely related to advancing maternal age and the risk of recurrence is usually small, between 1 and 2 per cent. Occasional cases of Down’s Syndrome (Mongolism) result from an extra chromosome 21 being attached to another chromosome (translocation). If one or other parent has the attached chromosome the risk of having a second affected child may be greatly increased. Before giving genetic counselling to parents who have had a child with Down’s Syndrome (Mongolism) chromosome studies on the patient or parents are appropriate at least where the mother was less than 30 years old when the patient was born. 22. ‘The examination of sex chromosomes has now an essential place in the investigation of primary amenorrhoea, male infertility and intersex states. (If there is doubt about the sex of a newborn infant specialist opinion should be sought immediately). TTurner’s Syndrome, characterised by undeveloped gonads, short stature, webbing of the neck with a low hairline, often cardiac and other anomalies, results from lack of one sex chromosome, ie there is only one X chromosome present giving a total of 45 chromosomes instead of the natural complement of 46. The incidence of the condition is about 1 in 4,000 female livebirths. These patients often present with amenorrhoea. Klinefelter’s Syndrome, which has an incidence of 1 in 1,000 male livebirths, is characterised by underdeveloped testes and