How is sex determined in various organism ? Give historical background of sex determination.
 

The mechanism of sex determination has always been a puzzle before the geneticists.

The initial clue about the genetic/chromosomal mechanism of sex determination can be traced back to some of the experiments carried out in insects.

The cytological observation made in a number of insects led to the development of the concept of genetic/chromosomal basis of sex determination.

Henking ($1891$) could trace a specific nuclear structure all through spermatogenesis in a few insects and it was also observed by him that $50 \%$ of sperm received this structure after spermatogenesis, whereas the other $50 \%$ sperm did not receive it.

Henking gave a name to this structure as the $X$-body but he could not explain its significance.

Further investigations by other scientists led to the conclusion that the $'X$-body' of Henking was in fact a chromosome and that is why it was given the name $X-$chromosome.

It was also observed that in a large number of insects the mechanism of sex determination is of the $XO$ type i.e. all eggs bear an additional $X$-chromosomes (autosomes).

On the other hand some of the sperms bear the $X$-chromosome whereas some do not.

Eggs fertilised by sperm having an $X$-chromosome become female and those fertilised by sperms that do not have an $X$-chromosome become males.

Due to the involvement of the $X$-chromosome in the determination of sex, it was designated to be sex chromosome and the rest of the chromosomes were named as autosomes.

Grasshopper is an example of $XO$ type of sex determination in which the males have only one $X$ chromosome besides the autosomes, whereas females have a pair of $X-$chromosomes.

These observations led to the investigation of a number of species to understand the mechanism of sex determination.

In a number of other insects and mammals including man, $X-Y $ type of sex determination is seen where both male and female have same number of chromosomes.

Among the males an $X$-chromosome is present but its counter part is distinctly smaller and called the $Y$-chromosome.

Females however have a pair of $\mathrm{X}$-chromosomes.

$\Rightarrow$ Both males and females bear same number of autosomes.

Hence the males have autosomes plus $XY$ while female have autosomes plus $XX.$