Question

Mental retardation in man is associated with sex chromosomal abnormality and is usually due to

• A. Increase in size of X-chromosome
• B. Increase in size of Y-chromosome
  
• C. Increase in number of X-chromosome
• D. Increase in number of Y-chromosome

Answer 1

The correct option is C Increase in number of X-chromosome

Sterile males with mental retardation, etc. are due to increase in their X-complement which takes place in a disorder called Klinefelter's syndrome. These are formed by union of an XX egg and a normal Y sperm or normal X egg and abnormal XY sperm. The individual thus has 47 chromosomes (44 + XXY)

So, the correct answer is 'Increase in number of X-chromosome'