AN article by Jessica Hamzelou in the February edition of New Scientist has some good news for men.
Research carried out in America suggests that ‘Adam’s rib’, the feeble Y chromosome which makes half of the human race male, is not withering away as some scientists feared. Examination of the genetics of a monkey relative seems to show that the little chromosome is much more resilient than was previously thought.
In mammals, including humans, an individual’s sex is determined by the presence or absence of a Y chromosome. Females have two X ones, so called because of their shape. Males have an X and a Y. Irrespective of which chromosome pairing we have, all of us begin life in the womb as females. If, however, a Y is present, a gene on it stimulates the development of testes. These in turn trigger the formation of the other male characteristics.
Originally, the Y chromosome had the same shape as the X. In creating egg and sperm cells, the X and the Y lined up beside each other and swapped strands of DNA, just as the other 22 pairs of chromosomes do. Swapping ensures that each new sperm and egg cell is unique; no two are genetically identical. It also acts as a DNA repair mechanism. Mistakes, known as mutations, occur in the process of cell replication. The exchanges of DNA when new sex cells are created help weed out such errors.
About 166 million years ago, according to geneticists, a freak mutation occurred in one of our mammalian ancestors; a large chunk of DNA was turned upside-down and reinserted in the Y. The damaged chromosome lost one of its arms in the process and became Y shaped. The X and Y no longer matched and, when they lined up to form new cells, genetic material could not be exchanged. The male descendents of this primordial ancestor have inherited the faulty Y intact. It has been transferred down from father to son ever since. With no input of DNA from the X side, the Y chromosome has been accumulating errors down through the generations. Originally, it would have carried 800 or so genes, just as the X does today. However, as mutations occurred the number fell. It now carries only 19.
The remaining genes are targets for further annihilation and some geneticists argue that it’s only a matter of time before they too are eliminated. This has already happened in at least one species of rodent. The Japanese spiny rat, for example, has no Y chromosome. In it, the functions formerly carried out by the Y have been dispersed to other genes. Mice lacking a Y were bred experimentally in an Adelaide laboratory by activating genes on other chromosomes to trigger the male pathway. Estimates, based on the rates at which mutations occur over time, have been made as to how long the human Y is likely to last. According a calculation which Hamzelou mentions, the Y will be around for another 4.6m years.
Research by Jennifer Hughes, of the Whitehead Institute at Cambridge Massachusetts, challenges these predictions. She has sequenced the Y chromosome in the rhesus macaque, a monkey found throughout central and southern Asia. It supports 20 genes which match the monkey’s X chromosome. Significantly, 19 of them are found in the human Y. This monkey’s ancestral line diverged from ours 25m years ago.
Only one Y gene, therefore, has been lost in 25m years. The loss of the other genes, she argues, did not occur gradually; they went when the original mutation occurred. The 19 surviving genes, she believes, have vital biological functions and can’t disappear.
Not all scientists agree. Hamzelou cites Jenny Groves of the University of Melbourne who maintains that versions of the 19 genes could, in time, migrate to other chromosomes. As they do so, the Y chromosome will continue on the path to redundancy.
Eventually, she believes, men will carry an empty space where the Y chromosome used to be. Either way, it seems, men are here to stay.
My condolences to radical feminist readers.
© Irish Examiner Ltd. All rights reserved