Sex Determination by Genes-Hormones-Environment

Gene balance theory of sex determination:

“Patterson” discovered that in Drosophila Y chromosome will not play in sex determination. A single dose of ‘X’ chromosome in a diploid individual will make it male. Two doses of ‘X’ chromosomes in a diploid individual make it female. “Bridges” proposed gene balance theory.

- Bridges gene balance theory: Ex Drosophila.

ccording to this theory the sex is developed basing on the male or female determining genes. If there is excess of female determining genes it develops into female. The ‘X’ chromosome in Drosophila carries more genes, that incline towards femaleness. The Autosomes will, possess genes which incline towards maleness. Hence in drosophila the sex Is decided by a ratio between X chromosomes and sets of Autosomes.

Quantitative theory of sex determination by Goldschmldt:

Gold Schimidth worked on gypsymath. (Lymantria dispar). This moth shows different geographical races. They occur in Europe. North America and Japan. In European species the sex factors are weak. In Japan races the sex factors are strong. In Gypsy moth maleness is carried, on chromosome, and factors for femaleness are seen in ‘Y’ chromosome and cytoplasm.


1) Goldschimidth’ crossed European weak female with ‘Japanese strong male. The result is all male off springs are normal. All the female offspring’s are intersex.

2) He crossed Japanese strong female with European weak male. The result is all off springs are normal.

If a European weak female with japanese strong male, one Japan race X chromosome can not totally dominate the female characters of and cytoplasm of European race, hence the daughter Is intersex,

Sex determination In Hadrocarbon Insect: Sex Is decided by the presence of Xa, Xb, Xc, Xci etc., alleles. Fertilized eggs will become females. Unfertilised eggs will become males. If closely related Insects take up sexual reproduction, the homozygous diploid males will develop’(all heterozygous diploid will become females)

Sex Determination-Hormones

ICrew’s sex reversal in chick: In case of chick the sex reversal is seen. If the ovaries are destroyed in the female chick slowly it becomes male. It develops testis, It produces male hormones. It becomes functional male. This is called sex reversal. It was indicated by Crew in chick This is called sex reversal. It was Indicated by Crew In chick. In mammals, fishes, amphibians, birds sex reversal Is noticed.

II) Free Martin Calf: Lithe found free martins in cow. When twins of opposite sex are born, the male is normal and the female is sterile. The female shows male characters. Such sterile female is called free martin. The male and female twins are attached to the same placenta In the mother womb. In the male offspring the hormones are produced first. These hormones through placenta will reach the female, because of which the female becomes sterile and more over it develops male characters. It is called free martin.


Environmental effect on sex determination: Ex :Bonellia

i) In Bonellia sex determination is by environmental effect. Bonellia

a marine, sedentary animal. The female Is big, it has a long probosis. The male lives as parasite In the uterus of female. The larva produced by the female are hermaphrodites. If the larvae move away from the probosis of female (the females of same species) they grow into female. Larva goes near the proboscis of any female of the species It becomes male.

ii) It is a marin’molluscan animal. If the larval form comes nearer the female, it develops into male. On the other hand if it goes away from the female, it becomes female.

Ex Creptidula - mollusca.


In an organism externally both female & male characters are exhibited, such organisms are called gynandromorphs. T. H. Morgan, C. B. Bridges, Gold schimidt observed gynandromarphs in Drosophila .& silk worms. Depending upon the female and male characters on the body external, three types of gynandromorphs are.recognised.

I). Bilateral gynandromorphs: In this type of flies, one of the lateral sides possess female characters & the other side possess male characters. Ex Drosophila, Silk worm.

ii)Antero postero gynandro morphs The organisms anterior side has male characters & posterior side has female characters. E.x Lucunus

iii) Sex Piebalds :This type of organisms mainly recognised as male or female. However the opposite sex characters are exhibited on & off on the body. Ex Drosophila.