Different Modern Theories those are Essential for Sex Determination are:
1. Chromosomal theory:
Sex of the individual depends on the behaviour of particular chromosome. The chromosomal mechanism of sex determination varies in different organisms.
In this method of sex determination both the sexes have got even number of chromosomes but female is homogametic having XX chromosomes while, male is heterogametic having XY. The female produces eggs with X chromosomes and male produces sperms of two kinds X and Y.
Drosophila has rod like chromosome X and its unlike met with hook at its end is Y (Sex chromosome) in male.
The rest chromosome i.e. 3 pairs (two V shaped or one dotted pair) which do not show sexual differences are called autosomes.
b) ZW method:
It is found in large group of animals, butterflies, moth, flies, birds and fishes. In this type, female is heterogametic and male is homogametic i.e. male has two similar chromosomes ZZ while female have two unlike chromosomes.
McClung found that in testes of grasshopper there were 11 pairs of autosomes and an odd chromosome with no mating. He further suggested that this odd chromosome was associated with sex determination.
Similarly, squash bug female was having 11 pairs of chromosomes while male had 10 pairs and odd chromosome.
Male produces two types of sperms one with X and other without X chromosome (heterogametic) while, female only one type of egg X.
Fertilization of egg by a sperm with 10 chromosomes will result into male. It differ from XY method only by absence of Y chromosome.
d) Honey bee method:
In this method, sex is produced at will in honey bee. Females are workers which are smaller while larger few are males called drones.
It develops partheno genetically (without fertilization) and therefore have a haploid chromosome number i.e. 16.
Queen bee and workers have 32 chromosomes. In this case, sex is determined by haploid and diploid constitution of chromosomes when female lays egg in worker cell sperms are emitted to fertilizer egg to develop into a female while unfertilized egg after hatching produces males.
2. Genie balance theory:
C.B. Bridges (1925) reported this theory of sex determination in Drosophila.
This theory states that
i) Genes for femaleness were located on X and for maleness on autosomes A.
ii) All individuals carry genes for both sexes.
iii) The ratio of number of X chromosomes; number of autosome sets in zygote indicate nature of sex irrespective of Y chromosome. The ratio is called sex index.
If it is less than 0.5 it produces male. If it is 1 or more it produces females, if it is 0.5 to 1 it produces intersexes, if it is more than 1.5 it produces super-female and if it is less than 0.5 to 0.33 it will produce super-male.
3. Quantitative theory:
In this theory sex is determined by the comparative ratio of X and Y chromosomes. Gold Schmidt while working with gypsy moth noted that there are different geographical races. The male is heterogametic XY and female homogametic XX.
The moths occur in Europe, M. Africa and N. Asia. Crosses between male and females of any one locality produce normal males and females.
But when strains of Japanese origin crossed with European produces progeny containing intersexes.
4. Metabolic differentiation theory:
Ridle concluded from his experiments on doves and pigeons that sex is determined by the degree of metabolism.
Less degree of metabolism causes the production of females while increased metabolism cause the production of males.
5. Hormonal theory:
Hormones are the secretion of the endocrine glands which are more conspicuous in the sex differentiation rather than sex determination.
The hormones coming from the ovaries and testes have a great influence on the sexual development. The chromosomes determine sex but further development of sex is influenced by hormones.
There is close relationship of sex glands (testes and ovaries) and sexual characteristics which appear in all parts of body.
A boy castrated before adolescence failed to develop beard, voice and other characteristics of male.
It is observed in many cases that the organisms are females for some part of their life and then turn to males or vice-versa. This phenomenon is known as sex reversion.
Sex is determined by hormonal balance e.g. Birds, Oysters and Marine Annelids. In birds, pituitary gland of female produces hormone, which causes development of ovary in the beginning but if female hormone is not produced ovary destroyed and testes like tissues may develop and enlarge and produces male hormone to cause reversal of sex.
Walker injected testicular extract from a rooster into hen, she developed a comb and showed characteristics of male and even attempted to mate other hen.
When twin calves of opposite sex are born the female is usually abnormal and sterile such calf is called free-martin sex.
6. Determination by environment:
a) In Bonellia, when larvae get attached to the proboscis of female develop into male but becomes female if settles down in mud and live in sea water.
b) In Mollusca, if young animals are rared alone become female, but if rared in contact of an adult they become males.
c) Whitney and Shull showed that in rotifers crowding of daphinds together were produces males.
d) Many flowering plants, earthworms and snails are hermaphrodites having genes for both sexes but sexes are expressed by separate part to the exclusion of others.
e) In Dinophilus, sex is determined by size of egg i.e. large eggs produce females and small ones males.
f) In Equisetum, females are produced by good growth conditions and males under poor conditions.