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Genetic Errors

 
The Embryo - Introduction
The Germ Cells
Germ Line and Somatic Mutations
Hermaphrodites.
Conception and Development.
The SRY Gene.
Genetic Errors.
Genetic Errors of Metabolism.
The Fetal Environment.
Some Statistics

 
During meiosis, errors sometimes occur. Sometimes only one of a chromosome pair is passed on, or sometimes multiple copies. When this occurs for one of the autosomes it gives pronounced developmental problems, and for most such occurrences the embryo does not survive. Errors of the sex chromosomes, however, seem to be more readily tolerated.

Babies are sometimes born as X0, XXX, XXY or XYY as well as a number of more complicated 'mosaics.'

While the autosomes are active in pairs, the X and Y chromosomes are not. That is to say, a male usually has only one X and one Y chromosome. In a female, only one of the X chromosomes in any given cell is active. Any 'surplus' X chromosomes are sequestered within the Barr Body. This may explain why missing or multiple copies are tolerated more easily.

It is estimated that as many as 1 in 1250 children are XXX. They grow as normal women, though they sometimes have menstrual problems. Many are fertile, and produce normal children. It has also been known for babies to be born with up to eight X chromosomes and a single Y, usually having considerable physical and mental problems.

They are a focus of attention, from time to time, in the scientific literature because of their genetic makeup, under the assumption that it proves gender identity is inborn. Others suggest that, because they are genetically different, they are not representative of people in general.

We describe here three of the most well-known syndromes. One wonders why XXX people are not better known. Perhaps it is because they are not often the subject of clinical literature.

Turner's Syndrome.

According to Bancroft,(1) one of the most common chromosome errors is Turner's Syndrome, where the child has only one X chromosome, that is, 45X0. Although the majority of fetuses are aborted, about one child in every 5000 born is affected. The baby develops to all appearances as a little girl, though often perceived as being very feminine in personality. However, although the embryonic ovaries start to develop normally, they soon regress. By birth there are no more than streaks of connective tissue. However, the external genitalia appears normal. The lack of ovaries means that there is a deficiency of sex steroids, and they may need hormone therapy.

Occasionally children are born as 45,X/46,XX, and develop in a very similar manner. It is thought that the second X is prevented from exerting its proper effect.

Klinefelter's Syndrome.

About 1 in 700 children are born as 47XXY, which is known as Klinefelter's Syndrome, where children develop along male lines. In severe cases the genital organs do not develop properly. They are clinically characterised as being tall and slender. There seems to be other physical and intellectual effects, but there is wide variation.

Many experience few problems until puberty, when they form a proportion, about five per cent of people attending infertility clinics. Bancroft suggests that the majority are free from what he refers to as stigmata, that is, features of behaviour and bodily appearance that set them apart.

This is not always true. The author's experience is that some are members of transvestite groups, and some are more seriously gender troubled. One person, who had escaped diagnosis until he was forty three, had cross-dressed when the opportunity arose, since childhood, and had always felt he should be a girl. At the time of speaking to the author, he was considering gender change but was having great difficulty in being taken seriously, in spite of his intersex condition. He had seen, he said, six different general practitioners and felt that they had simply laughed at him.

47XYY People.

The other common anomaly is 47XYY. Again these children develop along male lines. They are fertile and father normal XX or XY children.

On the radio the other day, I heard someone talking about XYY people as supermen, with extra intelligence and all the features that are supposed to be good in a man. This is a fable that I heard a number of years ago.

The most well-known study, in fact, claims to prove the opposite, but it is famous as an example of how things can go tragically wrong if a researcher doesn't select his subject group carefully enough.

It centred around high-security prisons and institutions in America, where it was discovered that there seemed to be an unusually high proportion of XYY men. The suggestion that XYY men might be more violent caught the attention of the press and, for a while, it looked as though totally innocent XYY people might be imprisoned 'just in case.' In fact, it turned out, on examining their records, that they were in high security prisons because of continual re-offending, but their crimes were civil ones, fraud and so on, not crimes of violence.

Subsequently a much more extensive survey in Denmark,(2) on young men entering national service, showed that XYY people are, in general, not outwardly remarkable from anyone else.

Their numbers are estimated at 1 in 700. Since XYY men grow up, have children and live their lives with no-one knowing that there is anything different about them, they would not be likely to attend clinics. It would seem to this author, that the above figure must have been derived from other studies. In other words, someone has gone out and looked for them, and no-one knows exactly how many there are.

Clearly the NHS only carries out such tests as are perceived to be necessary and karyotyping is not routine in British hospitals. In America where as much testing can be done as the parents are willing to pay for, it is becoming routine. Clinicians suggest that such babies will be more aggressive and less intelligent, something the people deny, once they are old enough to do so. Possibly negative stereotypes are lingering from the studies mentioned above and, given that it has not previously been seen to be a problem, the children are being pathologised unnecessarily.

Bibliography and good reading.

  1. Bancroft J. (1989) Human Sexuality and its Problems , Edinburgh: Churchill Livingstone. (bookshelf)
  2. Witkin H.A, Mednick.S.A, Schulsinger (1976) Criminality in XYY and XXY men, Science, 193, 547-555
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Attribution
Bland, J., (2000) About Gender: Genetic Errors.
http://www.gender.org.uk/about/04embryo/42_gnerr.htm
 
Book graphics courtesy of Amazon.co.uk
Web page copyright 1999-2006
Derby TV/TS Group. Text copyright Jed Bland.
15.05.98 Last amended 30.06.00, 22.09.10, 01.05.14