Nondisjunction of chromosomes

[Elliott]

Hello, i know that nondisjunction in chromosomes occur in meiosis II, when the sister chromatids fail to separate and both daughter chromosomes go into the same gamete.
My doubt is in this question where i think the correct one is b, but then again it doesnt specify meiosis II, still can be the correct answer?

Nondisjunction of chromosomes occurs in
a) mitosis when homologous chromosomes or sister chromatids fail to separate
b) meiosis when homologous chromosomes or sister chromatids fail to separate
c) mitosis when homologous chromosomes or sister chromatids completely separate
d)meiosis when homologous chromosomes or sister chromatids completely separate
e)none of the above

Thanks A lot,

[virlomi_stone]

Actually, non-disjunction can occur at meiosis I or meiosis II but it gives different types of gametes depending on which one it happens at. For example if a chromosome carries Aa then...
If normal segregation occured then the gametes would be:
Gamete 1:A
Gamete 2:A
Gamete 3:a
Gamete 4:a
If non-disjunction occurs at the first meiotic division for a chromosome that carries for example Aa then the resulting gametes would be:
Aa
Aa
O
O

O=nothing in the gamete

2 gametes would have an extra allele and 2 gametes would have no allele

If non-dishunction occurs at the 2nd meiotic division the the gametes would be:
AA
O
a
a

OR...

A
A
O
aa

1 gamete would have an extra allele, 1 gamete would have no allele and the other two would be normal.

So, you are right in selecting choice b but just be aware that non-disjunction does not occur at ONLY meiosis II but can occur at either and will give different results. The answer says homologus chromosomes OR sister chromatids because in meiosis I homologus chromosomes are failling to separate and in meiosis II sister chromatids are failing to separate.

Hope that helps!!

[sdekivit]

non disjunction can also occurs in mitosis when you add colchicine by preventing of the formation of the mitotic spindle. (but i't's an extreme case)

--> in this case: b is the correct answer.