Genetics - Meiosis

[shefv]

Which of the following is TRUE regarding meiosis?
A
Segregation of alleles and independent assortment of different gene pairs occur ONLY during the first meiotic division
B
Non-disjunction of chromosomes can occur in both meiotic divisions
C
Non-disjunction of chromosomes can occur in the first but not in the second meiotic division
D
The genetic exchange between homologous chromosomes can occur in the first or second meiotic division


I am confused - why is A not the correct answer?

Doesn't Segregation of alleles take place in Anaphase I because this is where the alleles have segregated into the haploid cell that they will now be a part of and independent assortment in metaphase I when the tetrads line up on the metaphase plate independent of one another. Meiosis II is just where the sister chromatids separate - segregation and independent assortment has already happened because they are haploid already.


The solution states that B is correct

But I am thinking that non disjunction of chromosomes happens only in meiosis I and it is the non disjunction of "sister chromatids" that happens in meiosis II (in which case C would make sense too)


Where am I going wrong with the concept?

---

Members don't see this ad.

 

[sc4s2cg]

Ah I did that question just yesterday.

The way I reasoned it:

It couldn't be A because it was way too narrow. First because of the ONLY statement. But then because it talks specifically about genes and alleles, when in context it should be about chromosomes or chromatids. And of course, segregation of chromosomes/chromatids happen in both metaphase I and II.

D is wrong because crossover only happens during prophase I.

And this is where I got confused. B is correct because nondisjunction of CHROMOSOMES can occur in mitosis 1, and of CHROMATIDS in meiosis 2. So nondisjunction can happen in either case and that's the point they wanted to get across.

Edit: also, remember that segregation in meiosis 1 refers to the tetrad. So the two chromosomes are very close together. In meiosis 2 the same thing happens except that the sister chromatids are very close together. This is how I rationalized after spending a good amount of time on that question.

 

[shefv]

And this is where I got confused. B is correct because nondisjunction of CHROMOSOMES can occur in mitosis 1, and of CHROMATIDS in meiosis 2. So nondisjunction can happen in either case and that's the point they wanted to get across.

By this statement - C would be right though. I also thought of it this way and B states that non disjunction of "chromosomes" takes place in both meiosis. But it is actually the nondisjuntion of chromosomes in meiosis I and nondisjunction of "sister chromatids" in meiosis II


Does segregation take place in meiosis I and meiosis II? I tried to search on this and could only find that it takes place in anaphase I.

 

[sc4s2cg]

This research article says that segregation occurs in meiosis II as well. Except in MII it results in equational division while in MI it results in reductional division.

Could you give me the question number and chapter that this question is in? As far as I remember, the answers were written in a way that assumed that 'chromosomes' in B refers to 'chromosomes + chromatids', in which case C is wrong (which is of course what I picked). This is a badly worded question. The takeaway is that nondisjunction can occur in both MI and MII, with chromosomes and sister chromatids respectively. Since you picked A, does the reasoning on why A is wrong make sense?

 

[shefv]

The question is from here:

http://www.mcatquestionaday.com/findquestion.php?arg1=2416

 

[supremus]

A
Segregation of alleles and independent assortment of different gene pairs occur ONLY during the first meiotic division

Explanation : False. Since separation of homologs and sister chromatid in meiosis constitutes the mechanical basis of Mendel’s laws.

B
Non-disjunction of chromosomes can occur in both meiotic division.

Explanation : Correct. This is the correct one since nondisjunction in meiosis I results from failure of homologs to separate and nondisjunction in meiosis II is the failure of sister chromatids to separate normally.

C
Non-disjunction of chromosomes can occur in the first but not in the second meiotic division

Explanation: False. Non disjunction can occur in the second meiotic division.

D
The genetic exchange between homologous chromosomes can occur in the first or second meiotic division

Explanation: False but interesting. Homologs separate during the first meiotic division but since nondisjunction is possible during meiosis 1 this can lead to an homologs pair in the secondary gametocyte. ** This homologs will not align next to each other and they will not recombine, the sister chromatids will just separate.