Question

Down's syndrome in humans is due to

• A. three 'X' chromosomes
• B. three copies of chromosome 21
• C. monosomy
• D. two 'Y' chromosomes.

Answer 1

The Correct Option is B

To solve this question, let's break down and analyze the given options regarding Down's syndrome:

1. Three 'X' chromosomes: This condition is referred to as Triple X syndrome, which occurs in females when they have three X chromosomes instead of two. This is not related to Down's syndrome.
2. Three copies of chromosome 21: Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. This leads to developmental and intellectual delays, and it is indeed the correct explanation for the genetic basis of Down's syndrome.
3. Monosomy: Monosomy refers to the presence of only one chromosome from a pair instead of two. This is not associated with Down's syndrome but can cause other genetic disorders such as Turner syndrome (monosomy X).
4. Two 'Y' chromosomes: This condition would result in an XYY karyotype, commonly associated with Jacob's syndrome, not Down's syndrome.

Based on the analysis above, the correct answer is that Down's syndrome is due to three copies of chromosome 21. This condition affects physical growth, causes characteristic facial features, and typically results in varying degrees of intellectual disability.