Genetics Q

[Deepa100]

This is for those of you that are genetics experts-
I always struggle with recombination because I don't understand how it will work for the sex chromosome case. I mean, X and Y can not recombine, right? What prevents this from happening?

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[tmatt]

Recombination occurs between homologous chromosomes, and the X and Y chromosomes are not homologous.
That would be my best guess...I haven't started MCAT studying yet, and haven't had Genetics in a few semesters

 

[Vihsadas]

Moving to the Study Q&A subforum.

It is my understanding that for Chromosomes that line up on the metaphase plate to recombine during meiosis I they have to have a region of homology to each other. The X and the Y chromosomes are not homologous chromosomes, however they do share a very small region of homology which could, in theory, recombine during Meiosis. I do not think that you will have to know that for the MCAT, but just know that homologous chromosomes can recombine, and non-homologous ones usually cannot. If they ask you a question about recombination between X and Y the information I just posted will most likely be in the passage. GL!

 

[Deepa100]

Moving to the Study Q&A subforum.

It is my understanding that for Chromosomes that line up on the metaphase plate to recombine during meiosis I they have to have a region of homology to each other. The X and the Y chromosomes are not homologous chromosomes, however they do share a very small region of homology which could, in theory, recombine during Meiosis. I do not think that you will have to know that for the MCAT, but just know that homologous chromosomes can recombine, and non-homologous ones usually cannot. If they ask you a question about recombination between X and Y the information I just posted will most likely be in the passage. GL!

Thanks!

 

[coffeebythelake]

This is for those of you that are genetics experts-
I always struggle with recombination because I don't understand how it will work for the sex chromosome case. I mean, X and Y can not recombine, right? What prevents this from happening?

Recombination occurs at the end of Prophase I where homologous alleles are swapped on homologous chromosomes. This is important for independent assortment, although Mendel's second law is an oversimplification of actual genetics.

That being said, X and Y do recombine at the pseudoautosomal region.

 

[tncekm]

Thanks!

And the reason that homologues recombine and others do not is that there has to be some high degree of nucleotide similarity for the two homologues to "stick" together. I.e. the homologous chromosomes are going to be almost entirely complimentary, with a few nucleotides here and there that are changed. If they weren't complimentary, the chromosomes couldn't stick together.

 

[Maxwell Edison]

They actually do recombine. Normally the effects are not noticeable, but in some cases you can get really weird/cool (depending on your perspective) mutations as a result of unequal crossing over.

Case in point: the SRY gene (the thing that makes men men) can be translocated onto an X chromosome, making you a sterile male with 2 X chromosomes (it's called XX male syndrome).

On the flipside, this can produce female offspring with a y chomosome, but obviously lacking the SRY gene.

 

[jimboapu]

X & Y do not recombine. translocation is different. This is out of scope for the MCAT anyway.

 

[Vihsadas]

It was my understanding that there was about 5% homology between the X and Y chromosomes. So while on the whole X and Y are thought of as unable to recombine, there is a section that can pair and recombine.

Here's a pubmed article that seems to support what the rest of us are saying:
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=138936
See the section titled "Gene Content of the Human Y Chromosome".

This is, however, out of the scope of the MCAT.

 

[coffeebythelake]

X & Y do not recombine. translocation is different. This is out of scope for the MCAT anyway.

Read my previous post. There is recombination in the pseudoautosomal part of the X and Y chromosomes. This is not translocation; they are homologous genes.

Here's a pubmed article that seems to support what the rest of us are saying:http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=138936
See the section titled "Gene Content of the Human Y Chromosome".

Good find.

This is, however, out of the scope of the MCAT.

Yep. The genetics component of MCAT is really easy.

 

[Zerconia2921]

Moving to the Study Q&A subforum.

It is my understanding that for Chromosomes that line up on the metaphase plate to recombine during meiosis I they have to have a region of homology to each other. The X and the Y chromosomes are not homologous chromosomes, however they do share a very small region of homology which could, in theory, recombine during Meiosis. I do not think that you will have to know that for the MCAT, but just know that homologous chromosomes can recombine, and non-homologous ones usually cannot. If they ask you a question about recombination between X and Y the information I just posted will most likely be in the passage. GL!

Recombination occurs during prophase I of mesios I.