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



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