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Name: Nathan
Status: Other
Grade:  Other
Location: CA
Country: United States
Date: May 2005


Question:
I heard this crazy guy on the radio today saying that all primate have 48 chromosomes (24+24) except humans, which have their second and third chromosome "fused" together? Is this valid?



Replies:
Accordong to the Primate Cytogenetics Network, there are 233 species of primates and their diploid numbers range from 16 to 60. Search Google using "primate diploid number".

Ron Baker, Ph.D.


Apes and humans differ by one pair is correct with humans having one less pair. Most scientists that study this subject favor humans fusing of two pairs to reduce the number and there seems to be evidence for this in chromosome 2. Some scientists favor the fission theory where apes got an extra pair from fission of one of their chromosomes...but I don't know of the evidence for this. By the way, primates closest to humans have 48 pairswhile others like monkeys differ substantially

PF


Yes, it is absolutely valid. You can see it if you can find a picture of the chromosomes (called a karyotype) of a chimpanzee and a human and put them side by side. It is assumed that in humans two chromosomes fused to produce a total of 46, because most gorillas also have 48. It makes more sense that the event happened in humans than that the event happened independently in the same spot at random in chimps and gorillas. WHen you look at a karyotype you note not only the number of chromosomes, but depending on how they were stained, you can see the pattern of bands (light and dark areas) on the chromosomes. When you take the two chromosomes that chimps and gorillas have, and put them end to end beside chromosome number 2 of humans, the banding pattern is almost identical. Click on this link to see a diagram of this:

http://www.indiana.edu/~ensiweb/lessons/chr.bk1.html

What you are looking at is a diagram of chromosomes 1-4 with the human chromosome on the left and the chimpanzee chromosome on the right. Note that for chromosome 2, humans have one long one, but the chimp has two short ones there. This also shows only one of each chromosome for each species, so when you add the other set, that gives humans 46 and chimps 48. Also note the similarities in the banding patterns between the two. They are very similar, but not identical, because time has passed since we last shared a common ancestor, and mutations have occurred in the DNA of both organisms.

vanhoeck



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