Down's Syndrome is also called Trisomy 21 because affected people have three copies of chromosome 21. How is this possible?

Question

a
In a mistake in meiosis, the fertilized egg has two copies of chromosome 21.
b
Either the sperm cell or the egg cell has one copy of chromosome 21.
c
In a mistake in mitosis, the fertilized egg has two copies of chromosome 21.
d
Either the sperm cell or the egg cell has two copies of chromosome 21.

Answer 1

The correct answer is:

d - Either the sperm cell or the egg cell has two copies of chromosome 21.

This occurs due to a mistake in meiosis, where the process of gamete formation results in one of the gametes (sperm or egg) having an extra copy of chromosome 21. When this gamete fuses with a normal gamete, the resulting fertilized egg has three copies of chromosome 21, which leads to Down's Syndrome, also known as Trisomy 21.