The document summarizes Calvin Bridges' theory of genic balance sex determination in Drosophila from 1926. The theory states that sex is determined not by XY chromosomes but by the ratio of X chromosomes to autosomes. If the ratio favors X chromosomes, the result is a female fly, and if autosomes outweigh X chromosomes, the outcome is a male fly. The exact molecular mechanism of this genic balance system remains unknown but studies suggest the X chromosome provides female factors and autosomes carry male determinants.

2. Department of Zoology
Government College
Kasaragod

5. Calvin Bridges

6. This theory was proposed by a Drosophilist Calvin
Blackman Bridges in 1926.
This theory fully explains the sex determination in
Drosophila.
This theory shows that the
Maleness. Autosomes
Femaleness. X chromosomes.

10. Sex determination in Drosophila is
quite different from humans
Sex in Drosophila is determined by
the ratio of number of X
chromosomes to that of the number
of sets of autosomes

11. D r o s o p h i l a h a s e i g h t c h r o m o s o m e s
( n = 4 )

12. 3 pairs
Autosomes

13. D r o s o p h i l a h a s t h r e e p a i r s o f
a u t o s o m e s a n d o n e p a i r o f s e x
c h r o m o s o m e s

14. Sex in Drosophila is determined by the
ratio of number of x chromosomes to
that of the number of sets of autosomes

22. The theory of genic balance
given by Calvin Bridges (1926)
1
states that instead of XY chromosomes,
sex is determined by the genic balance
or ratio between X-chromosomes and
autosome genomes.
2

23. • There are many suggestions regarding the exact molecular
mechanism genic balance system of sex determination in
Drosophila
• The exact mechanism is still unknown.Studies shown that the X
chromosome contains female determining factors
• Autosomes contains male determining factors
• Y chromosome in Drosophila does not have any role in sex
determination.

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