Turner's syndrome is caused due to the absence of ........ chromosomes.

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?

Three copies of chromosome-$21$ in a child with Down's syndrome have been analysed using molecular biology technology to detect any possible $DNA$ polymorphism with reference to different alleles located on chromosome-$21$. Results showed that out of $3$ copies,$2$ of the chromosomes of the child contain the same alleles as one of the mother's alleles. Based on this,when did the non-disjunction event most likely occur?

In humans,mental retardation is associated with sex-linked abnormalities. It is generally due to:

Which of the following disorders does not result in a sterile individual?

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