What is a change in the number of chromosomes called?

The exchange of chromosomal segments between two non-homologous pairs of chromosomes is known as:

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

Given below are two statements :
Statement $I$ : Down's syndrome is caused by the absence of one of the $X$-chromosomes.
Statement $II$ : Turner's syndrome is caused by the presence of an additional copy of the chromosomes.
In the light of the above statements, choose the correct answer from the options given below :

The trisomic condition of $Down's$ syndrome arises due to:

What is the number of Barr bodies in an individual with Klinefelter syndrome?