WILSON and Stevens.There are two types of chromosomes- Sex chromosomes and autosomes.The sex chromosome carries a gene that TRANSFERS from one generation to the next generation.Sex has differentiated the organisms producing eggs from those organisms producing sperms.The organisms producing eggs are KNOWN as females.The organisms producing sperms are known as males.Explanation-C.B Bridges proposed genic balance theory for sex determination in drosophila.According to bridges in Drosophila Y chromosome is HETEROCHROMATIN so it is not active in sex determination.In drosophila sex determination takes place by sex index ratio.Sex index ratio = Number of x chromosome/ Number of set of autosomes.The Drosophila gene of femaleness is located on x-chromosomes and the gene of maleness is located on autosomes.Thereby in Drosophila, sex determination depends on the X chromosome & autosome.Gene of male FERTILITY is located on y- chromosome and in drosophila, y- chromosome plays an additional role in spermatogenesis and development of a male reproductive organ, so y-chromosome for production of fertile male.X/A = 1 → female X/A = 0.5 → male(2A + XY) = Fertile male(2A + XO) = Sterile male.