Question

Sex determination in Drosophila melanogaster is based on

• A. XY mechanism
• B. Genic balance between X chromosome and autosomes
• C. Pseudoalleles
• D. Environment-chromosome interaction

Answer 1

The correct option is B Genic balance between X chromosome and autosomes

Genic Balance Theory was proposed by C.B. Bridges in 1922 for sex determination in Drosophila. It states that ratio of number of X-chromosome to that of the complete set of autosome determines the sex of Drosophila. According to this theory of sex determination, if the ratio of X chromosome to a total number of sets of autosomes (X/A) is >1, the organism will be super female. If the ratio of X chromosome to a total number of sets of autosomes (X/A) falls between 1 and 0.50, the genotype will show the intersex phenotype. The X/A value is 1.0 for normal female and 0.5 for the normal male. It clearly shows that in Drosophila autosome bear genes which incline for maleness and X chromosomes bear genes which incline for femaleness.

Pseudoalleles are the genes that functionally behave as allelic pairs but structurally they are different genes, not allelic pairs.

Human follows XY-XX mechanisms of sex determination wherein male is heterogametic and carries “XY” sex chromosome while female carries two copies of X-chromosomes (XX) as sex chromosomes.

Sex determination by environmental factor is seen in Bonellia. Presence or absence of bonellin, i.e., environmental factor determines sex in it. If larva develops free in water it becomes adult female. The larvae that are exposed to bonellin in the skin of an adult female develop into males.