Ask A Scientist Molecular Biology Archive

name Nathan 
status other 
grade other 
location CA 

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? 
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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.
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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
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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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