Down Syndrome and Nondisjunction

[cfive22]

There was a passage in TBR CBT 5 that talked about Down Syndrome. Now the question that confuses me is this one:

The process responsible for the majority of cases of Down's syndrome occurs during:

A. metaphase I.
B. metaphase II.
C. anaphase I.
D. anaphase II.

Answer: C is the best answer. Nondisjunction, the nonseparation of homologous chromosomes, is the event that is responsible for the majority of cases of Down's syndrome. The separation of homologous chromosomes usually occurs in meiosis I. Therefore, we can eliminate choices B and D. Furthermore, separation of chromosomes occurs during anaphase. Metaphase is that stage of cell division, in either mitosis or meiosis, where the homologous pairs of chromosomes line up at the equatorial plate. The best answer is C.

I picked D because that one was the more obvious one to me. Wouldn't anaphase II work as well as anaphase I? I thought nondisjunction in both anaphase I and II created some sort of trisomy condition...

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[MedPR]

edit: that didn't really explain it right. one sec.

 

[SaintJude]

Unless this is passage based question and the passage specifically states that trisomy is due to changes meiosis I, you are not expect to know this & the question is even wrong. Because indeed, trisomy (which leads to Down Syndrome) can be a result of nondisjunction in meiosis I and meiosis II. So you are so right, anaphase II would definetely work as well as anaphase I.

Nothing will explain it better than this video below:

http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.html

 

[cfive22]

Unless this is passage based question and the passage specifically states that trisomy is due to changes meiosis I, you are not expect to know this & the question is even wrong. Because indeed, trisomy (which leads to Down Syndrome) can be a result of nondisjunction in meiosis I and meiosis II. So you are so right, anaphase II would definetely work as well as anaphase I.

Nothing will explain it better than this video below:

http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.html

Okay I'm glad that it wasn't just me that thought that this occurred. I'm almost positive there was nothing in the text to point out that this would occur in meiosis I over meiosis II. If there was, the answer choice would have pointed to it as well. Thanks guys!

 

[pfaction]

Actually, I got that question too in TPR4.

In a male individual with Down's Syndrome (trisomy 21), how many chromosomes would be visible at metaphase I of spermatogenesis?

I have to go over at what points it's haploid and diploid. Apparently metaphase I is a diploid still, anaphase I would make it a haploid. So it'd be 47!
​

 

[chiddler]

Unless this is passage based question and the passage specifically states that trisomy is due to changes meiosis I, you are not expect to know this & the question is even wrong. Because indeed, trisomy (which leads to Down Syndrome) can be a result of nondisjunction in meiosis I and meiosis II. So you are so right, anaphase II would definetely work as well as anaphase I.

Nothing will explain it better than this video below:

http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.html

nice video

 

[SaintJude]

i still can't believe that i'm the pre-med on the social networking site that solely discusses mcat questions...Where people comment "nice video", instead of "nice hips" hahah. This is so funny. I"m not even on facebook or twitter, but i'm on MCAT Q&A. Happy studying...

 

[chiddler]

i still can't believe that i'm the pre-med on the social networking site that solely discusses mcat questions...Where people comment "nice video", instead of "nice hips" hahah. This is so funny. I"m not even on facebook or twitter, but i'm on MCAT Q&A. Happy studying...

there's still hope left in the world 😛

 

[MedPR]

Actually anaphase 1 is the correct answer. The question asks

"The process responsible for the majority of cases of Down's syndrome occurs during:"

If nondisjunction occurs in anaphse 1, you get two n+1 and two n-1 gametes. If it occurs in anaphase 2, you get one n+1, one n-1, and two n+0 (normal) gametes.

So two n+1 is more than 1n+1 therefore more likely. Even if that is too thin and nit picky for you, consider this.

Homologous chromosomes split in meiosis 1. Sister chromatids split in meiosis 2. Since trisomy 21 is a result of homologous chromosomes not splitting, it must occur in meiosis 1. It is incorrect to call the sister chromatids in meiosis 2 "homologous chromosomes."

 

[chiddler]

"Since trisomy 21 is a result of homologous chromosomes not splitting"

how do you know this? is this just a fact or is this reasoned.

you make a good point with the n+-1 numbers, though.

 

[SaintJude]

Ah, I see. The majority of cases. So you're saying b/c there's a 2:1 ratio of (n+1) cells in meiosis I NDJ: meiosis II NDJ, there's a greater likelihood of it occurring in meiosis I.

But what's to say that there are not some mechanisms that make nondisjunction in meiosis I less common than in meiosis II?

 

[MedPR]

Ok look at this. Here is what you get with nondisjunction in meiosis 2.
http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20II.htm

And here is what you get with nondisjunction in meiosis 1:

http://www.biostudio.com/d_ Meiotic Nondisjunction Meiosis I.htm


I'm not exactly sure, but I guess trisomy 21 means 3 different copies (colors)?

 

[SaintJude]

Oh yeah, I all too vividly remember how I agonized about the fact that trisomy is a case of aneuploidy NOT polyploidy in some other thread. I would actually prefer accurate representation instead of these video gimmicks.

In polyploid, there is a duplication of an entire set of chromosomes. And trisomy 21 is a form of aneuploidy because it has two copies of only Chromosome 21 (as opposed to an entire set of chromosome).

 

[MedPR]

Oh yeah, I all too vividly remember how I agonized about the fact that trisomy is a case of aneuploidy NOT polyploidy in some other thread. I would actually prefer accurate representation instead of these video gimmicks.

In polyploid, there is a duplication of an entire set of chromosomes. And trisomy 21 is a form of aneuploidy because it has two copies of only Chromosome 21 (as opposed to an entire set of chromosome).

Nice.

 

[chiddler]

and my question? were the animations supposed to answer them? could you please explain how, if so.

"Since trisomy 21 is a result of homologous chromosomes not splitting"

My question is how do you know that it's not sister chromatids not splitting?

 

[cfive22]

Just saw all these responses! Nice guys. It made me do some more research and I came upon this in ncbi

"Even though the maternal-age effect has been known for many years, its cause is still not known. Nonetheless, there are some interesting biological correlations. It is possible that one aspect of the strong maternal-age effect on nondisjunction is an age-dependent decrease in the probability of keeping the chromosomal tetrad together during prophase I of meiosis. Meiotic arrest of oocytes (female meiocytes) in late prophase I is a common phenomenon in many animals. In female humans, all oocytes are arrested at diplotene before birth. Meiosis continues only at menstruation, which means that proper chromosome associations in the tetrad must be maintained for decades. If we speculate that, by accident through time, these associations have an increasing probability of breaking down, we can envision a mechanism contributing to increased maternal nondisjunction with age. Consistent with this speculation, most nondisjunction related to the maternal-age effect is due to nondisjunction at anaphase I, not anaphase II.

The multiple phenotypes that make up Down syndrome include mental ******ation, with an IQ in the 20 to 50 range; broad, flat face; eyes with an epicanthic fold; short stature; short hands with a crease across the middle; and a large, wrinkled tongue. Females may be fertile and may produce normal or trisomic progeny, but males have never reproduced. Mean life expectancy is about 17 years, and only 8 percent survive past age 40."

So the reasoning isn't because it's a 2:1 ratio, but due to how primary oocytes undergo anaphase I increasingly later on in life.

 

[chiddler]

yes. but trisomy 21 can be caused by either parent. what you wrote just focuses on the mother.

plus look at the question from a testing question perspective not a research perspective lol

that's a really cool find though.

 

[cfive22]

yes. but trisomy 21 can be caused by either parent. what you wrote just focuses on the mother.

plus look at the question from a testing question perspective not a research perspective lol

that's a really cool find though.

True, but almost always occurs from the mother. Or that's what the passage told me. But yeah, makes you think in another way.

 

[SaintJude]

MedPr, is likely right applying the probability logic for this MCAT question. Whether in real life that's what happens...whatever, I'll wait until med school to find that out.

Even if he's not, I'll make peace w/ this question having learned 3 lessons:
Down syndrome (a case of trisomy) can occur through anaphase I and anaphase II
NDJ in anaphase I produce 2 aneuploid cells, but only 1 in anaphase II
Trisomy is a case of aneuploidy not polyploidy

And 4.) there is still hope in the world 😀..

 

[MedPR]

and my question? were the animations supposed to answer them? could you please explain how, if so.

"Since trisomy 21 is a result of homologous chromosomes not splitting"

My question is how do you know that it's not sister chromatids not splitting?

I'm not sure about what you quoted (I know I said it). It just came out as I was typing, so I guess I heard it somewhere before.

As SJ said, down syndrome is a case of aneuploidy, not polyploidy. Nondisjunction in anaphase 2 causes polyploidy, and in anaphase 1 causes aneuploidy. You are right that trisomy can occur from nondisjunction in either meiosis 1 or meiosis 2, but only meiosis 1 nondisjunction results in aneuploidy. So only anaphase 1 nondisjunction results in down syndrome.

Are we positive this is a discrete and not passage based?