With the immense variation observed in our natural world, more than one sex determination system exists. XY system, which exists in humans, is not even predominant. A few key examples tend to predominate: the ZW system in birds, XO in insects, haplodiploidy, and environmental sex determination systems. Insects are extremely diverse and occupy all sorts of niches. So it's not surprising that insects have many different systems of sex determination. Many insects have the XX/XY system that humans have . Some insects, such as Lepidoptera (moths and butterflies), have a ZZ/ZW sex determination system.

Some insects, such as some grasshoppers, have an XX/XO sex determination system. The insect with two sex chromosomes (XX) is female while the insect with one sex chromosome (X0) will be male.

This theory was established by the experiments of Calvin B Bridges. The early experiment of Bridges, though demonstrated that Y- chromosome is not important for determination of sex, it did not indicate whether X-chromosome alone determines the sex or if autosomes also play any role in the mechanism. Individuals could, however, be obtained, which had two X-chromosomes as in the normal female, but were intersexes. Each of these intersexes had an extra set of autosomes (A) indicating that autosomes play a definite role in determination of sex. In this connection, Bridges' experiments on intersexes and supersexes are of special importance. Bridges, as early as 1922, came across certain Drosophila individuals which were triploids and thus had three sets of chromosomes (3A + 3X). These triploid individuals were normal females and were crossed with diploid males (2A + XY). As is obvious, from such a cross, normal diploid males, triploid females, intersexes, supermales and superfemales were obtained. Chromosome constitution of these phenotypically different sexes had a definite bearing on the mechanism of sex determination

 The theory of genic balance given by Calvin Bridges (1926) states that instead of XY chromosomes, sex is determined by the genic balance or ratio between X-chromosomes and autosome genomes.

 The sex determination in Drosophila is quite different from humans. Drosophila has eight chromosomes (n = 4), three pairs of autosomes and one pair of sex chromosomes. Even though Drosophila possesses XX and XY sex chromosomal organization, unlike human beings, the Y chromosome does not have any role in determining the sex of individuals. The sex in Drosophila is determined by the ratio of the number of X chromosomes to that of the number of sets of autosomes. In simpler terms, the sex determination is achieved by a balance of female determinants on the X chromosome (X) and male determinants on the autosomes (A). This type of sex determination is called genic balance system.

The sex of an individual is determined by the ratio of the number of its X chromosomes and that of its autosomal sets. The Y chromosome does not play a role in the determination of sex in Drosophila. The ratio is termed as sex index and is expressed as: No. of X chromosomes = X Sex index= No. of sets of Autosomes Y What is Dosage Compensation?

Dosage compensation is the crucial process that equalizes gene expression from the X chromosome between males (XY) and females (XX). The genetic control of sex determination is often associated with dimorphic sex chromosomes). In the XY system, females are homogametic (XX), whereas males are heterogametic (XY). Although originally homologous to the X chromosome, the Y chromosome has degenerated over time, creating an imbalance in X-linked gene products between the two sexes. Dosage compensation mechanisms have evolved to equalize X-linked gene expression between males and females, thereby ensuring the appropriate balance of X- chromosomal and autosomal gene products in each sex Dosage compensation has been best studied in worms, flies and mammals, revealing three distinct strategies for equalizing X chromosome expression (Lucchesi et al., 2005) In Drosophila males, dosage compensation globally upregulates expression from the single X chromosome twofold. In one step, this strategy equalizes X-linked gene expression between males and females and restores the balance between the X chromosome and autosomes In mosaic individuals with XX and XO cells, the hyperactivity can be demonstrated in XO cells. The mutant and wild eye colour in both male and female flies develop with the same intensity. The enzyme level of several enzymes such as ^PGD (6-phosphogluconate dehydrogenase), G-6PD (glucose 6 phosphate dehydrogenase), tryptophan pyrolase and fumarase are found to be the same in both the sexes. In Drosophila, four autosomal male specific lethal genes are associated with dosage compensation. Their activity is regulated by master switch gene (SXI) that controls sex along X chromosome. It causes enhancement of genetic expression. The product of other three autosomal genes are required for MLE binding. The gene Sxl in XX flies turns on the tra gene, which leads to female differentiation. The Sxl gene also inactivates one or more males specific autosomal genes, mle. In XY flies, Sxl is not expressed and autosomal genes are activated causing enhanced activity of X chromosome.  No X-chromosome inactivation in female.  Hyperactivity of X-linked genes in male.  Discovered by A.S.Mukherjee.