Question

Calvin bridges stated that instead of XY chromosomes, sex is determined by the genic balance. What would be the sex of drosophila having XX + 3A and XY + 3A karyotype respectively?
A. Inter sex and super female
B. Metafemale and intersex
C. Intersex and normal male
D. Intersex and super male

Answer 1

Hint: Calvin Bridges in 1921 proposed the genic balance theory of sec determination in Drosophila. It states that the sex is determined by the ration of an autsomal allele that favours male behavious and alleles of the X-chromosome which favours femaleness.

Complete Answer:
In Drosophila the ratio of X-chromomses to the autosomal allele determines the sex. A wild type diploid male drosophila has 2 sets of autosomes and an XY chromosome and a wild type diploid female has 2 sets of autosomes and two X chromosomes.

According to the theory, the ratio between X and A is:
(I) 1.0 : Its a female
(II) 0.5: Its a male
(III) >1 : It's a super female
(IV) <0.5 : It's a super male
(V) Between 0.5 and 1 : Intersex

Situation in question	Number of X chromosome	Number of autosomes	Total number of chromosomes	X/A ratio	Sex
1.	2	3	11	0.67	Intersex
2.	1	3	10	0.33	Super male/ Meta male

According to the theory and and the situations given in questionthe sex of Drospjila in case of XX + 3A is intersex and in case of XY + 3A is super male.

Thus, the correct answer is option D) Intersex and super male.

Note: The Bridges results suggested that the male factors are localized on autosomes in Drosophila, female determining factors are located on X chromosomes. X/A ratio of 1 means that the sex of the fly is female and an X/A ratio of 0.5 means its a male.