Sex Determination

In earlier years many theories have been proposed by Hippocrates & also by Scientists for the determination of Sex of the offspring.Some of them are-

(1) The sex of the offspring depends upon Vigour of the parents in that way Vigourous fathers will produce Son and less vigourous father will produce daughter.

(2) According to Galen the Germ cells from the right Ovary of the mother produce male & from the left Ovary of the mother produce female.

(3) According to Professor Schenk Highly nourished mother produce daughter while ill fed mother produce son.

All these Postulations of Sex Determination are Purely of historical importance but with out any Scientific evidence.Further more three different theory have been expressed relating to determination of sex.They are-

(1) Progamic Theory- Sex Determination occurs much earlier than fertilization i.e as the gametes level.

(2)Syngamic Theory- Sex Determination occurs at time of fertilization.

(3)Epigemic Theory- Sex Determination occurs after the formation of zygote i.e after fertilization.

There are different modern theories of sex determination which are accepted till today.They are-

(a)Chromosomal Theory

(b)Genic Balance Theory

(c)Hormonal Theory

(d)Environmental Theory

Chromosomal Theory of Sex Determination

According to Karl Correns(1906) one of the sex of an individual produce '2' Types of gametes and each gametes determines a different sex on fertilization.In most of the organisms(including human beings) the females are Homogamatic and the males are Heterogamatic.Thus,the female produce Similar gametes i.e having similar chromosomes and heterogamatic male produce different gametes having dissimilar chromosomes.In this way the Chromosomal Theory of Correns is also known as the theory of Heterogenesis.On the basis of the Chromosomal Theory different scientists studied different method of Sex Determination in animals like

(i)XY Method

It is the most common method of sex determination studied by T.H Morgan on in Drosophila. This Fruit Fly has 4 Pairs of chromosomes.





Out of the 4 pairs,3 pairs are like chromosomes in both male and female i.e One dot shaped and 2 pairs V Shaped.But there is difference in structure in the last pair i.e the females contain a pairs of rod shaped chromosomes whereas in male One is rod shaped and the other is hook shaped named as Y chromosomes in male.These Single pair Unlike Chromosomes are only termed as Sex Chromosomes Where as rest other are called Autosomes.Thus,Drosophila has 3 pairs of autosome and one pair of sex chromosomes in its cells.During gametogenesis female germ cell every egg and polar bodies recessive similar chromosomes in half number of the total i.e 3 autosomes and a single sex chromosomes.

On the other hand each male gamete recessive 3 autosomes along with one X or One Y chromosomes.This indicate the females are homogemetic and males are Heterogametic.
So,the Y chromosome only appear in male individual in that way if the egg is fertilize by a sperm bearing Y chromosomes.A male fly will appear but if the sperm bearing X chromosomes fertilizes the egg A female fly will appear.In this way it is concluded that Y chromosome contains some factors which stimulate the genes for maleness.

This type of XY Method of Sex Determination is equally applicable to Homosapiens. In human being males and females carry 22 pairs of autosome and one pair of Sex chromosomes.Thus,genotype of human female is 44A + XX and the genotype of human male is 44A + XY. Thus,the sperm containing 22A + X chromosomes if fertilizes the egg result will be production a daughter but if the sperm containing 22A + Y chromosomes fertilizes the egg,the off spring will be a Son.But Recent Investigation have described that Y chromosome is an inert chromosome and plays vary little part in Sex determination.

(ii)ZW Method

In some Animals like Butterfly,Moth,Birds and some fishes the relation of sex chromosomes is just reverse of the XY method.In these animals the males are Homogametic while females are Heterogametic. Therefore in order to avoid confusion the letter Z & W are used in place of X & Y.Thus the males are ZZ and the females are ZW varieties.In chickens there are in totals 9 pairs of chromosomes.So that the cocks have the genotype 16A + ZZ and the Hens have 16A + ZW.So in this case the Sex of the offspring depends upon the nature of egg which is fertilize.

(iii)XO Method

In 1920 McClung has noticed the absence of one chromosome from the germ cell in testis of grasshopper.He found 11 pairs of Chromosomes with one odd chromosomes in males i.e the absence of Y chromosomes.In that way the Genotype of the female grasshopper is 22A + XX where as the genotype of male grasshopper is 22A + XO(Total 23 Chromosomes).Thus Here the presence of X chromosomes determines the sex of the individuals.In this case fertilization of an egg(A + X) with a normal sperm(A + O) results in male offspring(22A + XO).

This method also applicable to many other insects of Phylum Arthropod.This Method differs from the XY method only in the absence of Y chromosomes.Since it has described in Drosophila that the Y chromosomes has almost no role during Sex determination.The XO Method can said to be a Modification of the XO method by just neglecting the Y chromosomes.

(iv)Honey Bee Method(Diploid-Haploid Method)

The animals of the class Hymenoptera of Phylum Arthropod and other insects like Honey Bee,Wasp etc. show a very peculiar type of sex determination in case of honey bee.The bees have contains 3 forms of bees-one is queen,2nd are the workers and 3rd are Drones.Out of the 3 forms,the drones are only male insect in the colony and again they are haploid in nature because such males are produced Parthenogenetically i.e From unfertilized eggs.But the Queen and the Workers are produced by the normal process of fertilization and therefore both of them become diploid females,

(v)Sex Determination in Gypsy Moth

Gold Schmidth in 1955 experimented the Gypsy moth (Lymantria dispar).In these insects the males are genetically XX and females are XY. All are normally diploid but diploid intersex also appears in their colony.Mating among the moth of same geographic region results in the production of normal male or female But,mating among geographically different races,diploid intersex are produce.But,these intersex are also fertile.

                                Sex Determination