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11/23/2005
Some Genetic Histories
In the ongoing controversy over the teaching of so-called
intelligent design as an alternative to evolution, there were both
heartening and disturbing developments. Dover, Pennsylvania, a
town only 20-30 miles from Mechanicsburg, where I grew up,
voted overwhelmingly to oust a board favoring teaching of
intelligent design. Kansas went the opposite way. Last week, I
learned about my own personal evolutionary path. Some time
ago, I submitted a sample of my DNA to National Geographic’s
Genographic project and now have the results.
The Genographic project collects samples of DNA from willing
participants all over the world. The samples are analyzed in
order to compile a comprehensive genetic history of how we
modern humans spread out over the globe after emerging out of
Africa. To understand my own ancestors’ journey, I had to deal
with the term “haplotype”. I Googled the word (couldn’t find it
in my old dictionaries) and the simplest definition was roughly: a
set of closely linked genetic markers present on one chromosome
which tend to be inherited together. Other definitions use the
word “genes” instead of genetic markers.
For me, it’s easier to consider a haplotype as simply a long
section of DNA that gets transmitted from generation to
generation. I’m related to anybody else that has that same
haplotype in his or her DNA. DNA can be considered most
simply as a code consisting of essentially a long string of letters,
--CGATTGGCAAGT--, where A,C,G and T stand for the four
chemical compounds that are found in DNA. A haplotype is
then just a long, probably a very long, string of those letters that
gets passed from parent to child. A haplotype may contain
certain “markers”, particular letters or groups of letters, which
may mutate when one or more letters is replaced by another.
This mutation results in a new marker.
Last week, I logged onto the Genographic Web site and found
they had analyzed my Y chromosome. I’m a member of
haplogroup I, a bunch of humans sharing haplotype I, defined by
a marker known as M170. Haplogroup I is what Genographic
terms the final destination of a genetic journey that began around
60,000 years ago. According to my Genographic profile, every
non-African is a member of a lineage with a Y chromosome
marker known as M168. M168 traces back to a single African
man known as the “Eurasian Adam”. This Adam lived
somewhere between 31,000 to 79,000 years ago.
Descendants of Eurasian Adam moved out of west central Africa
up to North Africa and onto the Arabian Peninsula and Middle
East. About 20,000 years ago, the M170 marker appeared in the
Middle East and my haplogroup I ancestors bearing this marker
spread into southeast Europe. By that time, haplogroup I was
into hunting in groups and some of its members were known for
their art. After last week’s column dealing with voluptuosity, my
Genographic profile tells me that those early M170 artists were
notable for their voluptuous female carvings known today as
“Venus” figures. I was destined to write about voluptuous
women based on my genetic heritage!
Haplogroup I is now widespread throughout southeastern and
central Europe. This is consistent with the fact that my father
was of Pennsylvania Dutch extraction. I was 40 years old before
I found that “Dutch” was a corruption of “Deutsch” and realized
my heritage was German, not Dutch! Most of the early
Pennsylvania Dutch came from the German Rhineland,
according to my 1962 World Book Encyclopedia.
But enough of my DNA. A very brief article by Anne
Casselman (the author of the bra article cited in last week’s
column) in the December Discover magazine describes the work
of Matthew Binns, professor of genetics at the Royal Veterinary
College in London. Eclipse is the subject of Binns’ work. If the
name doesn’t ring a bell, Eclipse was a legendary racehorse in
Britain back in the 1760s and easily won all 18 of his races.
Binns has looked at DNA samples taken from Eclipse’s remains
to better understand racehorse development. Some 80 percent of
today’s Thoroughbreds count Eclipse as an early ancestor. Even
more striking is that 95 percent of today’s Thoroughbreds have Y
chromosomes originating with Eclipse’s great grandfather.
Binns also plans to work on a genetic map of inherited diseases,
the subject of an article by geneticist Dennis Drayna titled
“Founder Mutations” in the October Scientific American.
Drayna introduces the concept of “founder” mutations with a
discussion of the condition “hereditary hemochromatosis”.
People with this condition have an unusually high ability to
absorb iron. This might seem to be a good thing, especially for a
person who tends to be anemic. However, the condition actually
can lead to multiple organ failure and even death. At one time,
iron-deficiency anemia was life-threatening and this mutation in
the so-called HFE gene that governs iron uptake would have
promoted survival.
Where does the “founder” part come in? In Europe in the distant
past there was a lone individual who had this particular mutation
in the HFE gene. Today, there are an astounding 22 million
Americans carrying this mutation and all are related to this
distant person! Genetic detective work indicates that the
mutation originated between 60 and 70 generations ago. This
takes us back to around 800 A.D., about the time of the fall of the
Roman Empire. The most likely “founder” seems to be a Celt,
since the current prevalence of the mutation is concentrated in
Ireland, western Great Britain and Brittany in France. This
pattern matches the current Celtic pattern of distribution.
Interestingly, my Genographic profile cites the possibility that
the widespread distribution of my M170 marker is related to the
spread of the Celtic culture across Europe.
Now that the benefit of this founder mutation for iron absorption
is gone, why does the mutation survive? For the life-threatening
condition to develop, both parents must carry the gene. Carriers,
those with only one copy of the gene, tend to have more efficient
iron absorption, which can be a plus and is not life-threatening.
Only those with two copies of the mutation are in trouble.
Another, better known example of a founder mutation is that for
sickle cell anemia. A single copy of the mutated gene promotes
survival of malaria, so common in Africa and other tropical
climes. Two copies of the gene and you’re in big trouble with
sickle cell anemia. There are actually five known haplotypes of
mutations for sickle cell disease, with the five founders coming
from five regions of Africa, the Middle East and India.
How does a founder mutation differ from the mutations
responsible for various other diseases? Let’s compare it with
another type of mutation, the “hot-spot” mutation. To illustrate
the difference, let’s say that part of the normal sequence in a
gene is CATTGAATTC. Now let’s say that a founder mutation
is CATTAAATT. Here, A substituted for the normal G. In
patients with this founder mutation, the A-for-G mutation is
always surrounded by the same letters. In a hot-spot mutation,
there will be other sites, hot spots prone to mutation, in addition
to the A-for-G site, for example, CGTTAAATTA or
CCTTAAATTA.
Hot-spot mutations in a gene encoding a blood-clotting factor are
responsible for hemophilia. In this case, different mutations at
hundreds of locations in the gene sequence cause the same
disease. On the other hand, a particular form of dwarfism occurs
when a mutation occurs at one particular mutation-prone site in
the related gene. Both of these types of hot-spot mutations occur
spontaneously and patients with these diseases aren’t generally
related to each other as are those harboring founder mutations.
Today, the whole world is concerned about the possibility of any
type of mutation that would allow the bird flu to propagate via
human-to-human transmission. Enjoy that Thanksgiving turkey!
Allen F. Bortrum
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