MYSTERY OF Y CHROMOSOME

The presence of Y chromosome in an individual makes its owner male as it carries a male-determininng gene. Boys are born with one Y and one X chromosome in all their body’s cells, while girls have two X chromosomes. Uptil now it was believed that the two human sex chromosomes, X and Y originated a few hundred million years ago from the same ancestral autosome during the evolution of sex determination. The X chromosome has held onto some of its historical gene content but Y chromosome hasn’t mostly because they have no counter part to swap genes with and thus due to constant gene loss during evolution.

Recently whole human genome has been encoded. Before that, David Page director of the Whitehead Institute for Biomedical Research in Cambridge, Jennifer Hughes and others working with scientist at Washington University School of Medicine in St. Louis decoded the DNA sequence of the human Y chromosome and even chimpanzees Y chromosome. We all knew till today that human and chimpanzees are closest cousins but new researches have given out some contradict results. It was found that the genome sequences of human and chimpanzees are more than 98% homologous but their Y chromosomes are found to be only around 70% homologous which means they are 30% different from each other! Despite the close evolutionary link between human and chimpanzees, a comparison of the two species' Y chromosomes show a surprisingly vast number of differences between their two genetic sequences, according to an analysis published in Nature.

According to views presented in Nature the palindromes were discovered in 2003 which read same in both directions. They are said to play a prominent role in difference in genomes of the two cousins and also in preserving nature of single Y chromosome. Discovery of the palindromes explained how the Y chromosome has managed over evolutionary time to discard bad genes by the fact that it recombines with itself. Each palindrome readily folds like a hairpin, bringing its two arms together. The cell’s DNA control machinery detects any difference between the two arms and can convert a mutation back to the correct sequence, saving the Y’s genes from mutational decay. But the same palindrome protection system can turn disastrous when defects occur in normal functioning and can result in male anomalies . These palindromes also show that there is major difference in Y chromosome of human and chimpanzees mainly because of difference in their sequence in palindrome region. Chimpanzees have these regions wider and even larger (about 40%) than in humans. Most of these mirror image sequences also have multiple copies, unlike in human.

Other point which can be taken into account is that of mating habits. The chimpanzees are known to show polygamous relationships. Thus a single female is mated by many males which leads to the competition among the male sperms resulting into the the strongest one to win and thus the genetic information of strongest is passed on to next generation. This itself leads to rapid evolution of the Y chromosome.

Also, as Y chromosome does not have a counterpart to swap genes with during reproduction (except at very terminal ends were it attaches with homologous X chromosome), natural selection sees it as a single unit and any change in one of the gene will affect rest of the genes of Y chromosomes. This is not the case with other chromosomes as they can swap genes between the pair of chromosome before formation of egg and sperm.

"The Y chromosome seems to play by its own rules," said Willard. "This study really shows why we need to be looking at Y chromosomes in other organisms to figure out what exactly is going on."

The fast evolutionary changes which are taking place in Y chromosomes does not mean men evolve faster but it will affect the evolution process on a whole. But how these changes are affecting and the true mechanism behind this is still left to be understood and may take a long time as according to the record it took Page “13 long years to encode human Y and 8 years to encode chimp Y genome.”