It is well known that the Y chromosome, which is transmitted only to males and not females, is the sex-determining chromosome, and now research has revealed that this chromosome has many more hidden secrets than just the role of gender. Two independent studies have found that this sex chromosome was formed millions of years ago and still retains genes that have not changed, genes that are critical to male survival and whose absence or abnormal function would lead to disease or even non-viability. These findings may go a long way toward providing insight into why men and women have various differences in lifespan and disease occurrence, among other things. Andrew Clark, a geneticist at Cornell University, said classical textbooks generally portray that a few genes from the Y chromosome are fundamental in determining male sexual characteristics such as genitalia and beards. Research now suggests that this understanding is sorely lacking. Mammalian sex chromosomes have evolved over millions of years and were originated from two different chromosomes (which are completely different from other chromosomes), with males now having two different chromosomes, X and Y, and females having two X chromosomes. The presence or absence of a Y chromosome is a key determinant of gender, as the Y chromosome contains many genes that are essential for testicular development. Strangely, however, the X chromosome, which has evolved over time to exceed the length of the Y chromosome, still retains 2,000 genes, most of which were lost early in evolution and fewer than 100 of the original genes are now preserved on the Y chromosome. This phenomenon has led scientists to propose that this chromosome will gradually shorten in the future. In order to find how the genes on the Y chromosome have changed over the evolution of different species, biologists at the Whitehead Institute of Biomedical Research in Cambridge, compared the Y chromosomes of eight different mammalian species, including humans, monkeys, mice, cattle and wallabies, and found that not only do these genes determine the sex of the embryo, but that very similar genes exist between species. These genes have a wide range of functions, including controlling the expression of genes on many other chromosomes. Almost all species retain these genes, and although the Y chromosome sequences are very different, the genes that are retained are the same, suggesting that these genes are critical to the survival of these animals. Bellott carefully analyzed these genes and found that while single copies of those genes that determine gender are fine, these ancient genes must be double copies to maintain proper function. The study was published in Nature. The Y chromosome not only determines sex, which determines that males are more fertile and reproductive, but also determines the survival of males. This group plans to analyze the function of these genes in more detail in the future. A similar study was conducted in another article published in Nature, which used a different approach to gene sequence analysis and studied a different type of mammal, but also addressed questions about Y chromosome evolution. The conclusions of the study were similar. Now that the importance of the genes preserved on the Y chromosome to the genome as a whole is known, specific analysis of the function of these genes will provide greater insight into disease. This suggests that many traits in males are not only a matter of androgens, but that there are also factors at the level of gene expression. However, this study may suggest that there may be interactions between genes of species, especially between genes on different chromosomes, and that such effects are a key link in weaving together whole biological traits. the study of the Y chromosome may just provide a specific research tool.