Biology question down syndrome

[nihantokar]

Down syndrome can exist in two forms. Classic affects all cells and is very is severe the mosaic form rises from the presence of two or more cell types in the body with differences in chromosome number and structure. How does the mosaic condition arise?
a. non-disjunction of chromosomes in mitosis in early fetal development
b. non-disjunction of chromosomes in meiosis during the formation of ova
c. non-disjunction of chromosomes in meiosis during the formation of sperm
d. translocation of chromosomes at maturation of the ovum

the answer is C but I thought it should be A. can someone explain? thanks

[winter1]

Non disjunction can happen both in mitosis and meiosis, but the keyword in option A is fetal development.

I believe it’s early stages of mitosis’ nondisjunction that leads to Down syndrome instead of in fetal development.

[nihantokar]

thank you! but why is it the sperm not the ova?

[winter1]

Non disjunction in meiosis = Happen in gametes = Possible in both ova and sperm!

• If non-disjunction of chromosomes occurs during the formation of eggs (ova), and an egg with an extra chromosome 21 is fertilized by a normal sperm, it results in a fertilized egg with three copies of chromosome 21, leading to Down syndrome.
• If non-disjunction of chromosomes occurs during the formation of sperm, and a sperm with an extra chromosome 21 fertilizes a normal egg, it also results in a fertilized egg with three copies of chromosome 21, leading to Down syndrome.

[nihantokar]

thanks but im still confused, if both give down syndrome why the sperm is the answer?

[Aung_Htet_Thar]

[nihantokar]

thanks!!
from what I’ve understood this was a specific question for a specific answer…