nondisjunction

[pizza1994]

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If a gene on chromosome 21 is very close to heterochromatic DNA of the centromere, hence crossing over does not occur then in what meiotic divsion did the error occur in to cause down syndrome in the child?

Please explain! Thank you!

 

[wizzed101]

The description before the actual question is nonsensical because any gene on any chromosome near the centromere has a low frequency of recombination, and because during meiosis, all genetic materials are heterochromatic.

Secondly, there are different kinds of Down syndrome. Are we talking about the trisomy or the familial one? The former can occur in either meiotic divisions while the latter is just a result of genetic disposition.

 

[pizza1994]

The description before the actual question is nonsensical because any gene on any chromosome near the centromere has a low frequency of recombination, and because during meiosis, all genetic materials are heterochromatic.

Secondly, there are different kinds of Down syndrome. Are we talking about the trisomy or the familial one? The former can occur in either meiotic divisions while the latter is just a result of genetic disposition.

this was a question in my textbook that I was reviewing...answer was anaphase I but not sure why

 

[wizzed101]

I'm not sure about your book but the way that question was set up was meant to test you on the sequence of meiosis. Recombination takes place in prophase I and segregation takes place in anaphases. If "SOMEHOW" there was a nondisjunction resulted from recombination (or the failure of), then that nondisjunction event should take place in meiosis I.

And while it is true that recombination correlated to reduced chances of nondisjunction, we can't that the relationship is causative. Also, heterochromatin refers to the state of DNA in interphase. I don't believe it has anything to do with recombination.