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5th Sem Hons. (CC-12)

Sudeshna Ghoshal

Dept, of Zoology, VJRC.

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GENIC BALANCE THEORY

C.B BRIDGES (1921)

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CHROMOSOME THEORY OF SEX DETERMINATION

  • Male and females have 2 different sets of chromosome –
          • Autosomes (A)
          • Sex chromosomes (X, and Y).
  • Generally in Females, sex chromosomes are homo-morphic( eg. XX if X is designated as sex chromosome ).
  • While in Males these sex chromosomes are heteromorphic, ( eg. XY, where both are designated as sex chromosome)
  • [except in Birds where females are heteromorphic and males are homo-morphic ]

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The example below suggested that in Drosophila Y-chromosome does not help in determining sex.

2A + XX (Female) X 2A + XY (Male)

(non disjunction of X chromosome in female) male gametes

female gametes

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  • Y chromosome does not carry any sex determination factor,

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  • Presence of 1 or 2 X-chromosome is more important for Drosophila sp

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A+X

A+Y

A+XX

2A + XXX

Super female

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2A + XXY

female

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A+O

2A + XO

Male

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2A + Y

dies

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THE RESULT from EXPERIMENT of nondisjunction(1916)

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� GENIC BALANCE THEORY OF SEX DETERMINATION�

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  • C.B. Bridges ‘s experiment demonstrated that Y-chromosome is not important for determination of sex in Drosophila, again it did not indicate whether X-chromosome alone takes part in sex determination or Autosomes play any role in it.

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  • During experimentation in Drosophila he found individuals having 2 X-chromosomes but were intersexes. And each of these intersexes have extra set of Autosomes (A) indicating that autosomes definitely play some role ion sex determination.

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  • Bridges in the year 1922 came across certain Drosophila individuals which were Triploid females and has 3 sets of chromosomes (3A + XXX). These triploid individuals were found to be normal females.

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  • He crossed them with normal diploid males (2A + XY). From this cross he found normal diploid males and females, superfemales, supermales and intersexes.

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  • The presence of Triploid intersexes (3A + XX) proves that autosomes also play role in sex determination.

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THE EXPERIMENT

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A+X

A+Y

2A+XX

3A + XXX

Normal Female

X/A ratio=1.0

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3A + XXY

Triploid Intersex

X/A ratio=0.66

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A+X

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2A + XX

Normal Female

X/A ratio=1.0

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2A + XY

Normal Male

X/A ratio=0.5

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2A+X

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3A + XX

Triploid Intersex

X/A ratio=0.66

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3A + XY

Super Male

X/A ratio=0.33

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A+2X

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2A + XXX

Super Female

X/A ratio=1.5

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2A + XXY

Normal Female

X/A ratio=1.0

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3A+ XXX FEMALE

2A+ XY MALE

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According to this theory the ratio between number of X-chromosomes and number of complete sets of autosomes will determine the sex – i.e. a balance between autosome and sex chromosome.

  • The X-chromosome believed carry female tendency genes while autosomes carry male tendency genes.
  • X/A ratio determines maleness and femaleness in Drosophila.

If X/A ratio < 0.5 than individual will be - Metamale/Supermale (Sterile)

= 0.5 than individual will be Normal male (Fertile)

> 0.5 and < 1.0 yield Intersex/ Gynandromorphs (Sterile)

= 1.0 than individual will be Normal Female (Fertile)

>1.0 than individual will be Metamale/ Super-female (Sterile)

FINAL INTERPRETATION

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BRIDGES’ S PROPOSAL FOR DROSOPHILA IS -

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SUMMARY