The $Cri-du-chat$ syndrome is caused by a chromosomal structural change involving:

Select the incorrect statement regarding Down's syndrome.

An abnormal male child is born to normal parents. This child shows a short neck,stubby hands,slanting eyes,a permanently open mouth with a projecting upper lip,and short stature. This abnormality can be attributed to:

Match List-$I$ with List-$II$ and select the correct answer using the codes given below:

List-$I$ (Syndrome)	List-$II$ (Chromosomal abnormality)
$(1)$ Patau's syndrome	$(A)$ $44 + XXY = 47$
$(2)$ Klinefelter's syndrome	$(B)$ $44 + X = 45$
$(3)$ Down's syndrome	$(C)$ $46 + 1 = 47$,Chromosome $13^{th}$
$(4)$ Turner's syndrome	$(D)$ $46 + 1 = 47$,Chromosome $21^{st}$

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

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