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 mitosis, 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 meiosis, 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:

c. In a mistake in meiosis, the fertilized egg has two copies of chromosome 21.

Down's Syndrome, or Trisomy 21, typically results from nondisjunction during meiosis, the process of cell division that produces sperm and egg cells. Nondisjunction is when chromosomes fail to separate properly, leading to a gamete (sperm or egg) with an extra copy of chromosome 21. If this gamete contributes to fertilization, the resulting embryo will end up with three copies of chromosome 21 instead of the usual two.