Question

Mental retardation in man occurs due to:
A. Loss of one X chromosome
B. Additional of one X chromosome
C. Slight growth in Y chromosome
D. Overgrowth in Y chromosome

Answer 1

Normally, the general intellectual functioning is being expressed by the IQ (Intelligent Quotient). The World Health Organization suggested that when intelligent quotient is less than 70 and being accompanied with two or more key skills areas is defined as Mental Retardation (MR). It is generally defined before the age of 18 years.

Complete answer:
1. When it is seen that one X Chromosome is additional in male, the male chromosome structure becomes XXY. This is called Klinefelter Syndrome. In this disease, mainly the male becomes sterile. But there is no sign of mental retardation is seen due to this.
2. In mammals, the Y chromosome contains the gene SRY which is responsible for male development. As long as Y chromosome is considerably smaller than X chromosomes, it can’t be said the slight or overgrowth of Y chromosomes will be responsible for any syndrome.
3. X-linked mental retardation (XLMR) was defined as a monogenic intellectual disability affecting mostly males, having a higher prevalence of MR relative to females. So, we can say that loss of one X chromosome is linked with the syndrome of Mental retardation in male.

Hence the correct answer is option ‘A’.

Note: The mental retardation which is linked with X-linked forms are estimated to cause 10 to 20% of all inherited cases of mental retardation. The first gene to be identified was FMR1. This causes fragile X syndrome and still remains the commonest single gene abnormality to be identified.