Who reported that Down's syndrome is due to an extra $21^{st}$ chromosome?

Physical,psychomotor,and mental development is retarded in an individual affected with

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

Briefly explain Down's syndrome.

What would be the number of chromosomes in the ovum (fertilized by a normal sperm) that resulted in the appearance of Klinefelter's syndrome in the offspring?

In a diploid set of chromosomes,the deletion or addition of a chromosome leads to: