Jeebus
I'm reading a book about genetics, but I'm really confused about chromosomes. It's mainly vocabulary trouble. It is my understanding that the human genome has twenty-three pairs of chromosomes. I commonly hear people say that humans have twenty-three chromosomes, does that mean chromosome types? Because if there are twenty-three pairs, then that would make forty-six, right? The book I'm reading is called The Language of Genes by Steve Jones. Here is the particular passage that's messing with me:

"Every cell contains two copies of each of the chromosomes. The number is halved during a special kind of cell division in the testis or ovary. During the process, the chromosomes lie together in their pairs and exchange parts of their structure. Sperm or egg cells hence contain combinations of chromosomal material which differ from those in the cells of the parents who produced them."

Now, when I first started reading this, I initially thought that when it said "every cell contains two copies," it was referring to chromosome pairs (two chromosomes of the same type; one from mom, one from dad). Then it says it halves, so we're left with twenty-three independent chromosomes. Then it says that the chromosomes lie together in their pairs, so then I went back and re-evaluated what it meant when it said "two copies," since based on my initial understanding, after it split in half, there would be no more pairs. So, now, my understanding is that there are forty-six chromosome pairs (which divide into the twenty-three chromosome pairs in the process from the quote). Which would now bring the count of individual chromosomes up to ninety-two.

If I'm wrong (which I'm sure I am), please correct me. According to this picture (http://www.ncrtec.org/tl/camp/gene/male.gif), there are at least forty-six chromosomes (two times twenty-three). I'm totally confused by an apparent lack of clarity in what I'm reading (not clarifying whether it's about pairs of chromosomes or individual chromosomes, I'm just confused here).

http://biology.udayton.edu/MED/SCI230/LECTHELP-2/1DNA-intro.htm [Broken] - This is another source I was using to clear my confusion. This site further reinforces my idea that there are forty-six.

Can someone straighten me out here?

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Staff Emeritus
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Hi Jeebus, this topic belongs in Biology, but anyway:

A normal human cell contains 46 chromosomes. There are 22 autosomal chromosomes (which are identical in males and females) and they are numbered 1-22.

Everyone has two autosomal chromosomes of each type. One is passed down from the mother and the other is passed down from the father of the child.

Besides the two copies of autosomal chromosomes (total 44) everyone is supposed to have two sex chromosomes. Females have XX and males have XY. Thus giving you a total of 46 chromosomes.

There are diseases known where people have an abnormal number of chromosomes. Trisomy 21 (three copies of chr. 21) leads to the Down syndrome. Or, XXY leads to Klinefelter syndrome (poor secondary sexual development).

Jeebus
Originally posted by Monique
..Everyone has two autosomal chromosomes of each type. One is passed down from the mother and the other is passed down from the father of the child.

Besides the two copies of autosomal chromosomes (total 44) everyone is supposed to have two sex chromosomes. Females have XX and males have XY. Thus giving you a total of 46 chromosomes.

All right. For some reason, I was reading that passage a weird way and I think I get it now. Is the passage saying that as the chromosome pairs are divided, the two chromosomes exchange certain genes back and forth and whatnot right before the division, so the original orientation is lost? Are the X and Y chromosomes actually paired up? If so, why are certain traits always with the X and are never exchanged during this process with the Y? If not, do they just randomly join one of the gametes?

Staff Emeritus
Gold Member
Originally posted by Jeebus
Is the passage saying that as the chromosome pairs are divided, the two chromosomes exchange certain genes back and forth and whatnot right before the division, so the original orientation is lost?

The only thing wrong is the orientation lost. The orientation is not changed but the combination of genes on a given chromosome after the exchange (called recombination) is different.

Originally posted by Jeebus
Are the X and Y chromosomes actually paired up? If so, why are certain traits always with the X and are never exchanged during this process with the Y? If not, do they just randomly join one of the gametes?

Yes X and Y more or less paired up and recombination occurs for regions that are share by Y and X. Y is smaller than X and share some homology with X. Some traits are only with X because it is missing from Y and there no recombination in this case. The split up of the chromosome is more or less random. Half will get X and the other will get Y but it is not predetermined which harlf get what.

Staff Emeritus
Gold Member
Lets say you have two identical chromosomes which line up:
Code:
AAAAAAAAAAAAAAAAAAAAA
X   X
BBBBBBBBBBBBBBBBBBBBB
The two X's mark the place of recombination, the result is:

Code:
AAAAAAABBBAAAAAAAAAAA

BBBBBBBAAABBBBBBBBBBB
So in this example 3 genes were exchanged, but the number of genes and the identity of the genes is still the same. The only thing different is the origin of the genes (mixure of paternal/maternal genes).

Ian, I am very surprised that you say that recombinations occur between X and Y. How sure are you of that?

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