Down's syndrome occurs in the $21^{st}$ pair of chromosomes due to the presence of an additional chromosome.

What is the number of Barr bodies in a male with Jacobs syndrome?

The mechanism that causes a gene to move from one linkage group to another is called

Choose the correct answer regarding the following statements:
$(i)$ Sickle-cell anaemia is controlled by three pairs of alleles.
$(ii)$ Trisomy is a chromosomal disorder.
$(iii)$ $47$ chromosomes are present in a person having Turner's syndrome.

What is Down's syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down's syndrome increase if the age of the mother exceeds forty years?

An abnormal human male phenotype involving an extra $X$ chromosome $(XXY)$ is a case of