$A$ : $XO$ type sex determination is found in large number of insects.
$R$ : Some of the sperms bear the $X-$chromosome whereas some do not

Sex determination in Drosophila melanogaster is based on

Initial clue about the genetic/chromosomal mechanism of sex-determination can be traced back to some of the experiments carried out in

A woman has an $X$-linked condition on one of her $X$ chromosomes. This chromosomes can be inherited by

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.