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| Jaganshi (profile) wrote, on 4-10-2005 at 9:20pm | |
| Subject: http://education.guardian.co.uk/higher/sciences/story/0,12243,1448487,00.html |
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| Relax about sex selection Nature itself manipulates whether a particular couple has a boy or a girl, says Vivienne Parry. So fertility clinics won't be skewing population ratios by offering choice Thursday March 31, 2005 The Guardian News that MPs were proposing couples undergoing IVF could be allowed to select the sex of their baby was predictably greeted with horror in some quarters. Actually, the sex of offspring is already manipulated and has been for millennia - and on a much grander scale than anything offered by an infertility clinic. Sperm and egg are the only cells in the body that carry a single chromosome ration of 23. Reducing the normal 23 pairs to 23 singles means that, when the sex chromosomes are split, eggs will always get an X, but sperm carry either an X or a Y, depending on what gets divvied out to them. Thus in some senses, men decide gender. For if an X-bearing sperm does the filthy deed, it's a girl, if a Y gets there first, it's a boy. So absolutely random then. Er, no. Sex ratios vary substantially according to your partner, the environment, even when and how often you make love. Babies conceived in spring are more likely to be boys, as are those conceived during wartime. Male conceptions are more likely at the beginning or end of the fertile period around ovulation, whereas female conceptions are more likely in the middle. Then there's parental occupation. Abalone fishers (an abalone is a highly prized Australian mollusc that lives in deep water) and other divers, have more daughters as do professional drivers, pilots of high performance aircraft, anaesthetists and astronauts. Why? The theory - advanced in particular by Dr William James of the Galton Laboratory at University College London - is that it's parental hormone levels around the time of conception. High concentrations of testosterone and oestrogen increase the chances of having a son, and high concentrations of progesterone and luteinising hormone favour girls. The general rule is: more testosterone in women, and it's boys, less testosterone in men and it's girls. There is a theory - the Trivers and Willard hypothesis-that reproductive success is enhanced in parents who can manipulate the sex of their offspring according to their own condition. If times are hard - drought, lack of food - daughters are favoured, while when times are good, there is an investment in sons. The strategy is that your genes are more likely to be reproduced even by poor quality females, who are still likely to become pregnant, than by poor quality sons, who will be rejected and not beget offspring. Study of animal populations, such as caribou, back this up. Many human studies support this theory too. Recent work from the biological anthropology research group at the University of Kent showed that there is even an association between the sex of a woman's first child and what age they believe they might reach, their so-called subjective life expectancy. Among both affluent and deprived groups, it was women who were optimists and believed that they would reach a ripe old age, who were more likely to have had a son as their first born. The Trevors and Willard hypothesis doesn't always hold true. Times were hard during and after the second world war, yet war conditions irrefutably result in more boys, not girls. Why? Dr Valerie Grant, of the University of Auckland, believes that maternal dominance is key, a biological determinant that is underpinned by testosterone. Dominant women are likely to have higher circulating levels of testosterone, producing an excess of boys. Indeed an analysis by Grant of 353 women, successful enough to have made the pages of a national dictionary of biography, revealed a significantly high ratio of boys to girls among their children. Stress in women drives up testosterone levels, so this theory would also explain the anomalous war data. So there you go. Men wanting boys should have frequent sex with a dominatrix. Women wanting girls should smuggle in an anaesthetist for hot sex during ovulation. To be serious, it will be interesting to see, given the fact that more women have been killed by the tsunami, what effect this has on birth sex ratios in children born to the survivors. Against this background, imagining that human intervention on the trivial scale offered by pre-implantation genetic diagnosis, could have any serious or far reaching effect on human population, is anxiety too far. ยท Vivienne Parry is the author of The Truth about Hormones (Atlantic Books). |
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ChibiKeriana | 04-11-05 1:49am "So there you go. Men wanting boys should have frequent sex with a dominatrix. Women wanting girls should smuggle in an anaesthetist for hot sex during ovulation."
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