In XO type of sex determination

Explanation

Genic balance mechanism of determination of sex was first observe and studied of C.B bridges in 1921 in Drosophila. According to this mechanism, the sex of an individual in Drosophila melanogaster is determined by a balance between the genes for femaleness located in the X-chromosome and those for maleness located in autosomes, Hence the sex of an individual is determined by the ratio of number of its X chromosomes and that of its autosomal sets, the Y chromosome being unimportant.
Sex idex= `(" Number of X chromosomes")/("Number of autosomes sets")`
Individuals with sex index of 0.5 develop into normal males and those with sex index of 1 into normal females. If the sex index is between 0.5 and 1 the resulting individuals is called inter sex. Such individuals are sterile. Some flies have sex index of `gt 1` such flies are called super females or metafemales. Super males flies have a sex index value of `lt 0.5` and are also weak, sterile and non variable

Bridges drew, crossed a triploid females (3A+XXX) with normal diplod males (2A+XY) From such a cross he obtained normal diploid females, males , triploid females, intersexes, metamales and metafemales.