LONDON: Scientists have found that the first sex determining genes appeared in mammals some 180 million years ago. In humans and other mammals, the difference between sexes depends on one single element of the genome: the Y chromosome. It is present only in males where the two sexual chromosomes are X and Y whereas women have two X chromosomes.
A team from the Swiss Institute of Bioinformatics found that a very long time ago X and Y were identical until the Y started to differentiate from the X in males. It then progressively shrank to such an extent that now it only contains about 20 genes (the X carries more than one thousand genes).