Question

What is an inherited trait? ...

Why are we interested in inheritance?

What is the distinction between meiosis and mitosis?

How are genotypes and phenotypes related?

What are genes? ...

What is DNA? ...

What is the difference between DNA and RNA?

How similar are the genes of different species?

Answer 1

Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a

few insects, and it was also observed by him that 50 per cent of the sperm received this

structure after spermatogenesis, whereas the other 50 per cent 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-chromosome besides

the other chromosomes (autosomes). On the other hand, some of the sperms bear the X-

chromosome whereas some do not.

 Eggs fertilized by sperm having an X-chromosome become females and, those fertilized

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 the 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, XY type of sex determination

is seen where both male and female have same number of chromosomes.