The mechanism of sex determination has always been a puggle before the genetists.

Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a few insects and it was also observed by him that 50 percent of the sperm received this structure after spermatogenesis, whereas the other 50 percent could not receive this Henking gave a name to this structure as the X-body but he could not explain its sisgnificance Further investigations by other scientists led to the conclusion that the 'X body' of Henking was in fact a chromosome and that is why it was given-the name X-chromosome. It was also observed that in a large number of insects the mechanism of sex determination is of the XO type i.e. all eggs bear an additional X-chromosome besides the other chromosome (outsomes) On the other hand, some of the sperms bear the X-chromosome whereas some donot. Eggs fertilised by X-chiomosome becomes females and that fertilised by sperms that do not have an X-chromosome become males. Crassh copper is an example of XO type of sex determination. ln a large number of insects and mammals including man, XY type of sex determination is seen where both male and female have same number of chromosomes.

Among the males an X-chromosome is present whereas its counterpart is distinetly smaller and are called the Y-chromosome females however have a pair of X chromosome. Hence the males have outosomes plus XY and females have outosomes plus XX.