Question

Genic balance theory holds good in case of
a. Human
b. Drosophila
c. Grasshopper
d. Allium cepa

Answer 1

Hint: The theory said that the sex of the fruit fly determined by autosomal sets and number of the X chromosomes. The X-chromosomes carry female de­termining genes. The theory of genic balance was proposed by Calvin Bridges in 1926.

Complete answer:
Calvin Bridges in 1926 gave the theory of genic balance. He stated that the sex is determined by the genic ratio or genic balance between X-chromosome and autosomal genomes instead of XY-chromosomes. He worked on Drosophila.
To determine the sex in drosophila is different from that of a human being. Drosophila has three pairs of autosome and one pair of sex chromosomes. Drosophila possesses XX and XY chromosomal organisation. But in the case of drosophila Y- chromosome does not determine the sex of the individual. In drosophila, sex is determined by the number of X chromosomes to that of the number of sets of autosomes or in other words by the balance of female determinants on the X chromosomes and male determinant on the autosomes. When sex is determined by this type, is known as genic balance theory.
Hence, the correct answer is option (B).

Note: This theory also explains the reason for the occurrence of sexual variation in the population of the fruit fly. There are inter-male, metafemale and metamale apart from the male and female, these result from the difference in the ratio of $\dfrac{X}{A}$.
- If $\dfrac{X}{A}$ ratio is $1.00$ then the resultant individual is a female.
- If $\dfrac{X}{A}$ ratio is above$1.00$ then a resultant individual will be metafemale.
- If $\dfrac{X}{A}$ ratio is $0.50$, then the resultant individual will be male.
- If $\dfrac{X}{A}$ ratio is between $0.50$ to$1.00$ then a resultant individual will be intersex.