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 $XX.$
Initial clue about the genetic/chromosomal mechanism of sex-determination can be traced back to some of the experiments carried out in
Genetic identity of a human male is determined by
Two different types of gametes in terms of the sex chromosomes, are produced by
Select the incorrect statement
While solving the problem of sex determination in large number of insects, it was observed that