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 $XX.$
A woman has an $X$-linked condition on one of her $X$ chromosomes. This chromosomes can be inherited by
Male child will be born if
Initial clue about the genetic/chromosomal mechanism of sex-determination can be traced back to some of the experiments carried out in
Assertion : In humans, the gamete contributed by the male determines whether the child produced will be male or female.
Reason : Sex in humans is a polygenic trait depending upon a cumulative effect of some genes on $X-$ chromosome and some on $Y-$ chromosome.
Give differences : Male heterogamety and Female heterogamety