genetic question

[omarsaleh66]

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Can anyone explain in very simple terms how mosaicism or mosaic type pattern works? For example, Klinefelters (XXY) can be due to paternal nondisjunction or a mosaic type pattern. Just wondering if someone could tell me what that actually means? I've tried reading about it, but they use all these big words and i dont get it.

Later

 

[viper]

Sorry I was going to sit down, learn this and explain it, but gotta run. However, this looks good:

http://faculty.clintoncc.suny.edu/f...0 Lectures/Genetics- Human Genetics/human.htm

This looked good.

 

[HiddenTruth]

Can anyone explain in very simple terms how mosaicism or mosaic type pattern works? For example, Klinefelters (XXY) can be due to paternal nondisjunction or a mosaic type pattern. Just wondering if someone could tell me what that actually means? I've tried reading about it, but they use all these big words and i dont get it.

Later

Mosaicism is basically the presence of more than one genetically distinct cell type. So, normally all of our cells should be 46 XX/Y. But, diseases like klein's and turner's, where there is a huge in utero mortality rate, those that end up surviving are usually mosaics. So, some of their cells are 45;X (Turner's), while some are normal. 46,XX. This pattern of mosaicism can occur occas. with your trisomies as well. Hopefully that was a semi-elementary explanation.

 

[Ramoray]

Turners is an easy example. Mosaics usually occur when there are errors in cleavage of an ovum during early embryo. ( i dont know the damn proper terms in embryo. i hate embryo so excuse this) so anyway say an Ovum is XX hey tis a girl! woo, . so that ovum undergoes its first division to form 2 cells in embryogeneisis. If things go right 1xx ovum turns into 2 xx cells. But if thigns go wrong, like ananphase lag or nondisjunction during mitosis of this one ovum then something like this can occur:
1 xx turns into 1 x and a 1xxx cell ( the 1 xx duplicated its x chromosomes so it was 1xxxx before division, the nondisjunction caused 1 cell to 3x's and one to get 1x, so anyway now that 1 ovum turned into 1x and 1xxx cells. now if those cells both divide over and over smoothly in the end a human will have half of its cells with 1 x chrom. and half with xxx chrom. Now you will have some features of turners since half your cells are turner-like but since half are not you will not have full turnes, tehrefore your a mosaic cause you ahve some characteristics of turners.
Depending on which division goes wrong in cleavage you can have varyign degrees of mosaics likelike if the very first division went smoothly, and then one of the cells in the second division did nondisjunction than you could have 3 cell types in your body Normal XX, X, and XXX and then yould even have a diff set of characteristics. So mosaics usually occur in cleavage errors.

while on the topic, that is how the 3rd most commone( maybe 2nd ) way of getting downs snyd is. Mosaic of 21 is the same deal, with error in cleavage only its an nondisjunction of autosome 21 giving you some cells with 3 chrom 21s and some with 1 chrom 21 so you are mild downs. and this is not inheritible since it was a random error of cleavage.

 

[omarsaleh66]

wow, thanks alot guys. Great explanations and links.

later

 

[KittenKnows]

One of the things I was told about mosaicism is that in the normal female, only one X is active in the cell. The other X is inactive.
Human females inherit two copies of every gene on the X chromosome, (one from the mother and one from the father), whereas males inherit only one (always from the mother). If both X chromosomes were active within a given female's cell, it would throw things off as the genes on the X chromosome would be expressed at twice the rate in female cells compared to males. To avoid this, one X chromosome is randomly selected and inactivated and one remains functional (active) in each cell. So, the cells of females have only one functioning copy of each X-linked gene - the same number as males. The inactivated X can be seen as a dense, stainable structure, called a Barr body (after its discoverer). X inactivation is also referred to as a method of dosage compensation.
Recent studies (NATURE VOL 434 17 MARCH 2005) clarify the process: The X developed a way to inactivate - silence - most of the genes on one of the two Xs in females, so that males and females would in large part have the same dosage of gene products. Early in female development, cells randomly choose either the maternal or paternal X to be the active X chromosome. This choice is permanent, and has certain consequences. A famous example is the calico cat, the color pattern in its fur reflects the pattern of Xs inactivated in his or her cells during early development. Similarly, human females are mosaics of the X chromosomes from each parent, and the severity of an X-linked disease in a female depends on the percentage of the cells in which the mutant gene concerned is silenced or expressed.
http://members.shaw.ca/copingwithillness/xinact.html
There is much more information on mosaics from the above website. I think with the Kleinfelters, if the non-disjunction came from mom (2 x's), you could have a mosaic pattern (some of the x's will be inactive) and therefore have a "mostly" normal XY male. If the non-disjunction came from dad (X and Y), then you will not have X inactivation and will have XXY in all the cells. Don't hold me to this, you may still get X inactivation, I am not sure.

 

[omarsaleh66]

One of the things I was told about mosaicism is that in the normal female, only one X is active in the cell. The other X is inactive.
Human females inherit two copies of every gene on the X chromosome, (one from the mother and one from the father), whereas males inherit only one (always from the mother). If both X chromosomes were active within a given female's cell, it would throw things off as the genes on the X chromosome would be expressed at twice the rate in female cells compared to males. To avoid this, one X chromosome is randomly selected and inactivated and one remains functional (active) in each cell. So, the cells of females have only one functioning copy of each X-linked gene - the same number as males. The inactivated X can be seen as a dense, stainable structure, called a Barr body (after its discoverer). X inactivation is also referred to as a method of dosage compensation.
Recent studies (NATURE VOL 434 17 MARCH 2005) clarify the process: The X developed a way to inactivate - silence - most of the genes on one of the two Xs in females, so that males and females would in large part have the same dosage of gene products. Early in female development, cells randomly choose either the maternal or paternal X to be the active X chromosome. This choice is permanent, and has certain consequences. A famous example is the calico cat, the color pattern in its fur reflects the pattern of Xs inactivated in his or her cells during early development. Similarly, human females are mosaics of the X chromosomes from each parent, and the severity of an X-linked disease in a female depends on the percentage of the cells in which the mutant gene concerned is silenced or expressed.
http://members.shaw.ca/copingwithillness/xinact.html
There is much more information on mosaics from the above website. I think with the Kleinfelters, if the non-disjunction came from mom (2 x's), you could have a mosaic pattern (some of the x's will be inactive) and therefore have a "mostly" normal XY male. If the non-disjunction came from dad (X and Y), then you will not have X inactivation and will have XXY in all the cells. Don't hold me to this, you may still get X inactivation, I am not sure.

thanks alot, that really helped.

 

[joshua_msu]

I got a genetics question. It was from one of the NBME exams. Is the placenta composed of maternal chromosomes, or fetal chromosomes?


I didnt really now how to word it since I took the test a few days ago, but Im sure someone knows what Im trying to say.

 

[Pox in a box]

I got a genetics question. It was from one of the NBME exams. Is the placenta composed of maternal chromosomes, or fetal chromosomes?


I didnt really now how to word it since I took the test a few days ago, but Im sure someone knows what Im trying to say.

As the placenta and embryo are both derived from the blastocyst, the placenta is representative of fetal tissue. Maybe Dudek can back us up with why this is an important factoid.