Genic Balance Theory

In Humans, Y chromosomes determines the maleness irrespective the number of variants in X chromosomes.

So XX are females and XY are males

But if the Cytogenetics is XXY or XXXY then ​

What will be these individuals phenotypically ?​

Is this same method followed in Drosophila ??????

Chromosomal sex determination: Individual will possess specialized chromosomes called SEX CHROMOSOMES

Genic Balance Theory: Presence or absence of Sex chromosomes or relative ratio of these sex chromosomes will determine the sex of the organism.​

Example: Drosophila

In fruit fly Females are XX and males are XY but the mechanism of sex determination in Drosophila is different from Humans​

Drosophila has 8 chromosomes (n =4 )​

Three pair of autosomes and one pair of allosomes (Sex chromosomes)​

XX- females and XY- males but Y doesn't determine maleness like it does in humans​

Sex in drosophila is determined by ratio of ​

X chromosomes : Autosomes.  

BINGO- Genic Balance Theory​

Sex determination in Drosophila is based by striking the balance between the female determinants on the X chromosome and male determinants on the autosomes.​

This theory is called as Genic balance theory, was proposed by Calvin Bridges in 1926​

This theory also explains about the variants like intersex, meta males and meta females in fruit flies.​

It is based on X/A ratio where X is number of X chromosomes and A is no of autosome sets​

If ratio =1 it is a females​

Files will be Males when the ratio is 0.5​

Between 0.5 to 1 are inter-sex flies (shows both male and female characteristics)​

If the ratio is greater than 1 than the flies are meta females (several developmental problems​)

If the ratio is lesser than 0.5 than the flies are meta males (sterile and weak)​

HOW DO WE CALCULATE........?​

Generally the flies may have one X or two X chromosomes and Two sets of autosomes ​

So if there are 2 X then 2/2 (X/A) = 1 ….... Females​

And If there is 1 X then ½ (X/A) = 0.5 ......... Males