Question

Describe the genetic balance theory of sex determination.

Answer 1

It is a thoery of sex determination given by Calvin Bridges in 1926, determined in Drosophila melanogaster (fruit flies). Because of this Genic Balance System Calvin is able to explain the resons for sexual variations in fruit fly population such as inter-sex, metamales, metafemales. Sex determination in Drosophila is different from human beings.

Complete answer:

Definition- the theory of genic balance which was given by Calvin Bridges states that unlike in human beings instead of XY chromosomes, here sex is determined by ratio between X-chromosomes and autosome genomes i.e sex is determined by ratio of number of X-chromosomes to that of number of sets of autosomes.

Description- As stated sex determination in Drosophila is different from humans. However in Drosophila is has 8 chromosomes i.e 4 pairs of chromosomes having one pair of sex chromosomes i.e XY and XX and three pairs of autosomes. But here in this case the Y chromosome does not have any role in determining the sex of individuals Further in simpler words to explain determination of sex is obtained by balance of female determinants on the X-chromosomes(X) and male determinants on the autosomes (A).

Sex determination and Results:
As sex is determined by ratio between X chromosomes and Autosomes (A).
Following conditions are seen in sex determination:
- If ratio X/A is 1.00, the individuals will be females.
- If ratio X/A is 0.50, the individuals will be males.
- If ration X/A is between 1.00 and 0.50, the individuals will be intersex (having characteristics of both sexes).
- If ratio X/A is above 1.00, the individuals will be metafemale.
- If ratio X/A is below 0.50, the individuals will be metamales (these are sterile and weak).
So, based on these results sex determination in Drosophila is decided and is explained. This is called Genic theory of sex determination.

Note: The sex determination is based on two types that is, Chromosomal sex determination and Non genetic sex determination. In humans and Drosophila sex determination is chromosomal determination that is based on sex chromosomes.