Under the Microscope: A recent book (Adam's Curse: A Future Without Men, Phantom Press, 2003) by Brian Sykes, Professor of Human Genetics at Oxford University, predicts that the human male is doomed to extinction within the next 125,000 years, writes Prof William Reville.
Sykes has a well-founded international reputation in his field, but, other equally well-qualified scientists disagree with his conclusions.
Human genetic material is based on the chemical DNA. The DNA is organised into structures called chromosomes, and every cell in our bodies contains 23 pairs of chromosomes. One chromosome in each pair came from your father and the other chromosome came from your mother (homologous chromosomes). One pair of these 23 pairs determines your gender - the sex chromosomes.
There are two types of sex chromosome, the X and the Y chromosome. The human female contains two X chromosomes, and the human male contains one X and one Y chromosome. The Y chromosome contains the genetic material that confers maleness.
The long strings of DNA in the chromosome are divided into working stretches of information called genes. These genes are the units that specify our inherited characteristics. Under a microscope it is noted that each of the two chromosomes in every pair of the 23 pairs in the human female look very similar. In the human male, this similarity applies to only 22 pairs of chromosomes. When the Y chromosome is compared with the X chromosome it appears short and stunted. Correspondingly, each of the chromosomes in our cells contains 1,000-odd genes, with the exception of the Y chromosome which contains fewer than 100 genes.
Each chromosome in a chromosome pair contains genes for the same characteristics inherited from the father and the mother. The genetic information in DNA resides in sequences of chemical units called nucleotides. All chemicals in our cells suffer damage through "wear and tear" during the course of the cell's activities, and DNA is no exception. However, because the information in DNA is so important, the cell has elaborate mechanisms to repair damage (mutations) to DNA.
The primary mechanism is called re-combination. This basically means that damage on a chromosome can be replaced by undamaged DNA from the homologous chromosome. Unfortunately the Y chromosome has no equivalent homologous chromosome with which to exchange information and repair itself. It is therefore particularly susceptible to damage through genetic mutations.
The Y chromosome is active only in the testes, but it is frenetically active because cells must divide very quickly to produce millions of sperm cells every day. Every time a cell divides the DNA must be copied and every time DNA is copied there is a possibility of making a mistake. So, Y chromosomes in the testes are particularly susceptible to mutation and cannot repair these mistakes through re-combination.
Sykes claims that "a staggeringly high number of infertile men are infertile because of mutations in the Y chromosome" and that this can lead to a drop of 0.1 per cent in the fertility of the whole population every generation. He extrapolates that over 5,000 generations, within 125,000 years, male fertility will be just 1 per cent of what it is today. With each successive generation, fewer functioning Y chromosomes would emerge, setting a downward spiral towards Y chromosome extinction.
Sykes denies that loss of the Y chromosome necessarily means the end of the human race. Human reproduction could continue through cloning.
In Sykes's opinion, society would be better off without men. He reckons that sexual selection, the evolution of male characteristics designed to attract women, accounts for most problems in the world, such as wars and violence.
Sykes's views are not shared by most of his colleagues. An Australian group has calculated that the Y chromosome loses about five genes every million years and at this rate will last another 10 million years. Dr Robin Lovelle-Badge, a leading geneticist, points out that the damaged Y chromosomes in the 1 per cent of men infertile because of this problem are not passed on to the next generation, whereas the Y chromosomes that are passed on are perfectly capable of propagating both sexes.
Another researcher, Dr David Paige, recently showed that Y chromosomes have some capacity for self-repair. My UCC geneticist colleague, Dr Tom Moore, points out that there are mammals (voles) that have lost the Y chromosome already, but who still produce males. So, the basic premise of Syke's argument - that loss of the Y means loss of males - is demonstrably false. As the current Y is lost, other chromosomes or genes probably take over the sex determining function.
The Y chromosome has existed for a few hundred million years. Evolution works slowly and gradually and it seems most unlikely to me that it would make such a drastic step as the destruction of maleness in a 125,000-year period, a blink of an eye in evolutionary time. Also, to conclude that an all-female society would be relatively trouble-free is, in my opinion, naive.
William Reville is associate professor of biochemistry and director of microscopy at UCC.