Chromosomes and Sex Differentiation

The age-old search to discover the cause underlying the development of the two sexes seemed to be crowned with success at the onset of the twentieth century. The discovery of an unlike pair of chromosomes in the spermatocytes of different species of insects led McClung (1902) to suggest that the two types of spermatozoa might give rise to male and female individuals respectively. Like many new ideas, McClung’s was not immediately accepted. Even three quarters of a century later, when the chromosomal mechanism of sex differentiation is firmly established in many classes of animals, some of the early problems still remain unsolved.