What is Genic Balance Theory or Sex Index ratio?
Genic Balance Theory, proposed by Calvin
Bridges in 1921 based on his work with Drosophila melanogaster,
suggests that sex is not determined by the mere presence or absence of a Y
chromosome, but by the numerical ratio between the number of X chromosomes
and the number of sets of autosomes (A).
Watch our simplified video here: Genic Balance Theory
According to Genic
balance theory
- Y chromosome in Drosophila does not determine sex. It is only required for male fertility.
- Sex is determined by the balance between female-determining factors on the X chromosomes and male-determining factors on the autosomes.
- The fundamental formula is the Sex Index Ratio:
- X/A Ratio= No of X chromosomes/No. of Sets of Autosomes
Classification of Ratios and Phenotypes
Based on his experiments with polyploid flies (flies
with extra sets of chromosomes), Bridges categorized the resulting sex based on
specific ratios:
|
X:A Ratio |
Resulting Phenotype |
Chromosomal Makeup (Examples) |
|
1.00 |
Normal Female |
AAXX (2/2) or AAAXXX (3/3) |
|
0.50 |
Normal Male |
AAXY (1/2) or AAXO (1/2) |
|
Between 0.5 and 1.0 |
Intersex |
AAAXX (2/3 = 0.67) |
|
Above 1.00 |
Superfemale (Metafemale) |
AAXXX (3/2 = 1.5) |
|
Below 0.50 |
Supermale (Metamale) |
AAAXY (1/3 = 0.33) |
In summary
- X chromosome carries genes that promote femaleness
- Autosomes carries genes that promote maleness
- “Genic balance” refers to the balance between these opposing genetic factors
What is the role of Y chromosome?
