When one or more genes are missing from the middle part of a chromosome,it is called:

The number of chromosomes in Klinefelter's syndrome is

In $Drosophila \text{ } melanogaster$, $3$ pairs of autosomes and sex chromosomes of $XO$ type give rise to a normal male in appearance. In humans, $22$ pairs of autosomes and $XO$ type of sex chromosomes result in:

Down's syndrome is caused by the trisomy of which chromosome pair?

What is the cause of $Down's$ syndrome?

The "cri-du-chat" syndrome is caused by a change in chromosome structure involving: