Re: Change from 48 to 46 chromosomes

Sean Stinson (
16 Oct 1995 00:58:49 GMT

Todd replied to Sean,
: Or have two ape chromosomes joined to form the human chromosome 2? Most
: explanations tend to present such things from the human perspective. Do
: all apes have 48? If so, then its a lot more likely for the human
: condition to be the innovation, rather than the ape lineages (gorilla,
: chimp, p.chimp) to have independently done the same thing. What is the
: condition of the DNA in question in the orang? in gibbons?
Sean followed up with,
I should correct myself, two chromosomes of the great apes
have fused to form human chromosome 2.(Hedrick,P.W.,Weaver,R.F.(1992)
Genetics 2nd. Ed.,Wm.C.Brown Publ.,pp101)
I'll post the whole paragraph since it may be of interest to others:
"...much research has focused on human chromosomes and on those of
the related great apes. Humans have a diploid chromosome complement of
46, while chimpanzees, gorillas, and orangutans have 48. The difference
in number is explained by a centric fusion of two nonhomologous
chromosomes in the great apes to form human chromosome 2. Two
acrocentric chromosomes in the three great apes are homologous to the
metacentric human chromosome 2, and have identical banding patterns,
except in the fusion region and in the terminal heterochromatin.
A number of other structural differences exist in the chromosomes
among these four species. For example, the banding patterns in
chromosome 3 in humans, chimpanzees, and gorillas are virtually
identical, while the patterns in orangutans differs by two inversions.
The break points of the larger of these inversions is pericentric and
changes the chromosome from metacentric to acrocentric. Evidence
indicates only one translocation difference among these species. it
occurs between chromosome 5 and 17 in the gorilla and is not present in
the other species. Overall, the banding patterns for humans are nearly
identical to chimpanzees for 13 chromosomes, to gorillas for 9
chromosomes, and to orangutans for 8 chromosomes. Almost all the same
bands are present in these species, suggesting that the primary
chromosomal differences among them are in the order of the genes, not in
the presence or absence of genes."(same as above ref.)
I thought I'd throw in a short glossary for the laymen out there:
ACROCENTRIC-a chromosome with its centromere toward on end.
CENTROMERE-constricted region on the chromsome where spindle fibers are
attached during cell division.
HETEROCHROMATIN-chromatin that is condensed
and inactive.
HOMOLOGOUS-chromosomes that are identical in size, shape except allelic
differences and genetic composition.
PERICENTRIC INVERSION- an alteration in the sequence of genes and centromere
in a chromosome.
TRANSLOCATION-movement of a chromosome segment from one chromosome to
another nonhomologous chromosome.
*above glossary stolen from noted text.*