triploidy and trisomy???

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wiscrn
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triploidy and trisomy???

Post by wiscrn » Sun Jun 03, 2007 4:07 pm

HI, I am so confused between the two. OK, so triploidy is the addition of a chromosome (at the gamete level)???? so, instead of having 1 chromosome (X), meaning 2 strands to form the chromosome, there are 3? is this right?
Uhgggg! pleas help!

:?:

Darby
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Post by Darby » Sun Jun 03, 2007 7:52 pm

I believe triploidy is addition of a whole set of chromosomes, and trisomy is just one extra chromosome, which can happen in gametes (that's the most discussed version) or during production of somatic cells.

From the second part of your question about strands doesn't really make any sense - I think you're confusing the chromatids visible on most depictions of chromosomes with the functional chromosomes themselves.

wiscrn
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Post by wiscrn » Tue Jun 05, 2007 12:58 pm

You may be right. I'm confused with just the basic make-up of the darn chromosome???? :oops:
In the books, I keep seeing the karyotype, which shows a paired set of chromosomes lined up next to each other. what is thisrelationship??? Are they jusy grouped on b/c of their size? or is one chromosome from dad and one from mom (or that mixture already on the gene, and scattered throughout the chromosome?...please help....I can't go further wthout the basics... :wink:
Thanks for any input!

Darby
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Post by Darby » Wed Jun 06, 2007 12:24 am

They're paired because each carries codes for the same genes (the term "homologous" means "same information"), which is why each in a pair is the same size; each is part of a set that came from one parent or the other.

wbla3335
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Post by wbla3335 » Wed Jun 06, 2007 8:09 am

Hi wiscrn,

Perhaps I can clarify things a bit. The karyotypes you see in the books are the paired chromosomes, as you suspected. In diploid organisms, like us, we get one set from our mother and one set from our father. So eggs and sperm contain half the number of chromosomes (23) that our body cells contain (46). When the sperm fertilises the egg, each half set pair off when the cell divides, make copies of themselves, and one copy of each chromosome then goes to each of the two daughter cells. These strands you mentioned are probably the chromatids that Darby mentioned. During replication before cell division when the chromosome pairs come together, each individual chromosome must make a copy of itself. So in a chromosome pair, you temporarily have 4 chromosomes - the maternal chromosome and its copy and the paternal chromosome and its copy. When the cell actually divides, these get pulled apart so that each daughter cell gets one copy of the maternal chromosome and one copy of the paternal chromosome, ending up with two copies of each chromosome in each cell. Humans have 23 such pairs of chromosomes in each body cell, so 46 chromosomes in all. A triploid has a complete extra set of unpaired chromosomes. So if a triploid human were possible, it would have 23 + 23 +23 = 69 chromosomes in each body cell. A tetraploid would have 23 + 23 + 23 +23 = 92 chromosomes. Polyploidy (having more than two copies of each chromosome) is fairly common (not in humans!!) in the natural world, especially in plants.
Karyotypes from different species, unfortunately, don't all follow the same rules. I believe that human chromosomes are numbered according to size, plus the X and Y chromosomes. We often arrange and number the chromosomes from other species differently.
Trisomy is when there is a third copy of one of the chromosomes. For example, Down's syndrome is caused by the presence of an extra copy of chromosome 21. So in a person with Down's syndrome, each body cell has 2 copies of chromosomes 1-20, 22, and sex chromosomes, but three copies of chromosome 21 for a total of 47 chromosomes (46 + 1).
I hope this helps.

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