Select the incorrect statement regarding Down's syndrome.

Mental retardation in humans,associated with sex chromosomal abnormality,is usually due to:

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 loss of a chromosomal segment is due to

Match Column $I$ with Column $II$ and find the correct answer.

Column $I$	Column $II$
$(A)$ Aneuploidy	$(i)$ Increase in whole set of chromosomes
$(B)$ Monoploidy	$(ii)$ Loss or gain of a chromosome
$(C)$ Polyploidy	$(iii)$ Two sets of chromosomes
$(D)$ Diploidy	$(iv)$ $A$ single set of chromosomes

Edward's syndrome,Patau's syndrome,and Down's syndrome are due to: