Question

A mother is afflicted by Down's syndrome caused by an extra copy of chromosome $21$. Father is normal. Percentage of offspring affected by the disorder would be

• A. $100\%$
• B. $75\%$
• C. $50\%$
• D. $25\%$

Answer 1

The correct option is C $50\%$

Mongolism or Down's syndrome is an autosomal disease caused due to the trisomy of the 21st chromosome as a result of non-separation of chromosomes. Trisomy 21 can result from non-separation of the chromosomes in the mother, non-separation in the father, and after the egg and sperm have merged. It is a chromosomal disorder so is neither dominant or recessive. As, it is an autosomal disorder, so it does not matter whether the male parent is affected or female parent. Thus, if the mother is affected, then there is 50% chance of having offsprings with Down syndrome, as two types of gametes produced by affected mother (one with the normal number of gametes and other with an abnormal number of gametes).

So, the correct answer is '50%'.