 |
 |
|
|
 |
|
|
|
|
|
 |
|
|
Your Caveman Gramps Had Ants in His Pants
|
|
|
A genetic mutation that causes hyperactivity
could have been crucial to human development.
|
|
THE
PAPER: Evidence of positive selection acting at the human
dopamine receptor D4 gene locus
THE JOURNAL: Proceedings of the National Academy
of Sciences, January 8, 2002
THE AUTHORS: Yuan-Chun Ding, Deborah L. Grady, James
M. Swanson, and Robert K. Moyzis, et. al.
THE GIST: A genetic mutation that causes hyperactivity
could have been crucial to human development.
BEFORE TRANSLATION: "Based on an observed bias toward
nonsynonymous amino acid changes, the unusual DNA sequence organization
and strong linkage disequilibrium surrounding the DRD4 7R allele,
we propose that this allele originated as a rare mutational
event that nevertheless increased to high frequency in human
populations by positive selection."
Scientists believe that some of the bad genes that we humans
walk around with today are there for a good reason. By "bad
genes," they mean the ones that predispose young people to disabling
diseases and disorders; by "good reason," they mean an evolutionary
advantage. If these genes were entirely bad, say scientists,
they would not be so common.
That's the spirit in which biologist Robert Moyzis and colleagues
from the University of California at Irvine studied attention
deficit hyperactivity disorder. The ailment is linked to a variation
of a gene called DRD4. This gene makes a receptor for dopamine,
a biochemical that in addition to conveying feelings of pleasure,
also helps people focus on specific tasks. About 50 percent
of kids diagnosed with ADHD have the gene variation, which is
present in about 10 percent of the population worldwide. Moyzis
wondered: How could a gene that makes it almost impossible for
people to concentrate be so widespread? At some point during
our evolution, he decided, the gene must have given its bearers
a fantastic edge. But what could the advantage have been?
Moyzis and his team decided to look at the gene closely, and
what they found surprised them. There are more than 10 different
versions of the DRD4 gene, but by far the oddest is the one
associated with ADHD. By analyzing its structure and comparing
it with the other versions, they concluded that the variation
must have occurred suddenly, rather than evolving over many
years. What's more, says Moyzis, "the genes adjacent to the
variant DRD4 were almost exactly the same, whether in an individual
from Africa, America, or Europe." Since genes constantly rearrange
themselves and mutate at a certain rate, an ancient variation
should have been surrounded by many different combinations of
genes among different people. Consequently, he reasoned, the
gene must have sprung up sometime in our recent history.
|
|
|
|
 |
|
|
