

The 44 Chromosome Man - russell
http://www.thetech.org/genetics/news.php?id=124

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bluekeybox
This is really no big deal biologically. Think of DNA as the actual code and
of chromosomes as the modules/files that hold it. The man in question still
has all the "code" that a normal human has except some of those modules got
accidentally concatenated. It is sort of irrelevant to individual survival
(except for reproduction/future generation) how the code is organized as long
as it is present in its entirety.

Similar events happen all the time. I worked at a clinical genetics lab for a
while, and some tests that were performed (FISH analysis) tested for whether a
fragment of one chromosome is "embedded" within another. The term is
chromosomal translocation. Apparently there are quite a few translocations
that are completely benign and happen all the time. Usually the bigger the
translocation, the higher the chance that something goes wrong. However, when
an entire chromosome is translocated, the boundaries of one of the chromosomes
remain intact, so there is no surprise that it happens.

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2mur
Pretty interesting.

Incidentally, I discovered that I actually have a balanced whole arm
translocation of chromosomes 5 and 16. That is I have one normal 5, one normal
16, and two abnormal chromosomes one with a 5p/16q and one with a 5q/16p. I
only found it out when I rotated through a cytogenetics lab and we dropped our
own chromosomes for grins. It's not a Robertsonian translocation (doesn't
involved acrocentric chromosomes) but it's right through the centromeres of 5
and 16. Pretty weird, but as far as I can tell, totally asymptomatic. I'm a
physician (and hobby-python guy) and have 4 completely normal kids.

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tocomment
Wait so how could you have Children?

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2mur
Yeah my oldest kid was slow to talk (fine now) so his pediatrician wanted to
make sure there wasn't anything wrong, so his chromosomes were done (as well
as Fragile X and all the other usual suspects). The report came back as normal
chromosomes, so I can only assume he got my normal 5 and 16 (or that my wife
was stepping out!!).

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mrb
Will this person have a possibly slightly longer maximum lifespan due having 4
fewer telomeres thanks to the 2 fused chromosomes? It is known that telomere
shortening is responsible for aging.

Is there some sort of weak correlation between the lifespan of a species and
its number of chromosome?

And is it possibly the reason why 46-chromosome human ancestors prevailed over
48-chromosome human ancestors?

(I am no biologist; these are probably stupid questions.)

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thret
Quoting from article:

What scientists don't know yet is how 46 chromosomes came to win out over 48.

There is almost certainly no advantage in how the DNA is packaged. If
anything, having a different number of chromosomes hurts someone in terms of
having the most babies possible.

It might be that where the two chromosomes fused together, some new helpful
gene was created. Unfortunately, there is no evidence to support this.

An alternative is that dumb luck allowed the 46 chromosome humans to win.

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blahedo
This is interesting because it provides a mechanism for speciation that
_doesn't_ involve sequestration in a lost valley or something like that: in an
already established population, once you have a herd or tribe that has
translocated chromosomes floating around, that subgroup has much-lowered
chances of (successful) mating outside the subgroup. That, in turn, means that
any beneficial mutation that arises by random chance is kept "in-house", as it
were, and refined in the crucible of inbreeding, while the subgroup begins to
succesfully outcompete the rest of the local population—and that local
population can't acquire the beneficial mutation because of the breeding
problem.

Keep in mind the very large numbers we're dealing with when we talk about
speciation events. All of recorded human history is perhaps six or eight
thousand years, but a _hundred_ thousand years is a short time frame on an
evolutionary scale; so even an incredibly rare mutation will happen many
times.

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icedpulleys
While translocation is cool, this certainly isn't a novel case and
Robertsonian translocations are covered in undergrad genetics courses. Well
written if not for the way that it starts off, making it sound like the doctor
in China has made a unique discovery.

<http://en.wikipedia.org/wiki/Robertsonian_translocation>

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jinushaun
Humans with strange number of chromosomes. Ah... Takes me back to my
undergraduate genetics course. This definitely not as unique as the article
makes it sounds, and it's not surprising that he appears normal given the
nature of the genetic event.

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kmm
I wonder how many people harbour strange genetic surprises like this. How many
of you have had a karyotype made, let alone a complete DNA analysis?

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killerswan
Exactly. Right now we're only beginning to explore this stuff.

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Scriptor
It's interesting to consider how the ancestors of modern humans who first had
46 chromosomes became dominant. It could have been due to simple genetic
drift, maybe the family with the translocation had a high number of offspring
by chance for generation after generation, until it became a significant
portion of the population of the early hominids.

More likely, (in my opinion), is that the translocated chromosome itself
contained some beneficial alleles (gene versions) that gave its carriers an
edge. This has nothing to do with the translocation itself, it just so happens
that that particular chromosome could have a version of a gene (or versions of
several genes) that increased the carrier's fitness in the environment. Some
of theories regarding early hominid evolution cite a movement away from
heavily forrested regions into more open grasslands. Maybe this early
translocated chromosome contained alleles that were beneficial in the
savannas.

At this point you'd have a number of individuals with one copy of the
translocated chromosome and 47 total chromosomes (meaning the other homologous
(<http://en.wikipedia.org/wiki/Homologous_chromosome>) chromosomes are not
joined together, "normal") and a number of people who still had the original
48 chromosomes. As the article mentions, only 1/3 of the pregnancies of a
48-chromosome individual with a translocation individual would survive.
However, if there was inbreeding present individuals with the translocated
chromosome could be mated with each other. Here we have an even _less_ likely
chance of a successful pregnancy (8/36 or only 25%) but on the other hand we
now have the possibility of individuals with _just_ the translocated
chromosome. Instead of a joined chromosome and 2 unjoined chromosomes, these
individuals have 2 joined chromosomes and a total of 46 chromosomes.

Now remember that I postulated that the joined (translocated) chromosome could
have carried beneficial alleles as opposed to other, unjoined versions. Some
of those alleles may be recessive, meaning they'd be eclipsed by other
alleles. For an individual with two copies of the translocated chromosome they
would only have those recessive alleles, so they wouldn't be as likely to be
eclipsed. Again, these recessive alleles may have brought about beneficial
traits that weren't expressed in individuals who only had one joined
chromosome. Thus, individuals with 46 chromosomes could have had an even
higher chance of survival and finding other 46-chromosome individuals, thus
continuing this trend.

Anyway, I admit that much of the above is really just guesswork so take it
with a grain of salt. I'm primarily exploring the possibilities that could
have resulted in the original 48->46 chromosome shift. It was probably a mix
of genetic drift, having beneficial alleles, and the founder effect
(<http://en.wikipedia.org/wiki/Founder_effect>). Hope you enjoyed it!

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pyre
How likely would it be that there was a near-extinction event and the
reduction in the gene pool somehow caused this?

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ggchappell
Interesting. I wonder if this fellow might reasonably be considered a new
species.

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bendmorris
"His chromosomes are arranged in a stable way that could be passed on if he
met a nice girl who had 44 chromosomes too." So yes, he would.

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gort
"Species" is a vague and slightly philosophical concept, but I think an actual
population of people with 44 chromosomes would have to become established
before the term was applied.

"A species" is typically defined as something like "a group of individuals
that can breed with each other".

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hugh3
So if we discovered the last Tasmanian Tiger out in the wild somewhere then it
wouldn't count as a species any more because it's the last one and... aww,
this is too nitpicky a semantic debate even for me.

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zorked
The definition is obviously about potential: the last healthy Tasmanian Tiger
could mate with another healthy Tasmanian Tiger and have baby Tasmanian
Tigers.

A healthy mule can't mate with another healthy mule and have baby mules.

People who happen to be sterile or decided not to have kids can remain a part
of the human species...

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pash
> As the 44 chromosome man shows, this sort of change starts out pretty rare.
> Right now having this particular set of chromosomes is probably a 1 in 7
> billion sort of thing.

Guffaw.

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pash
Those of you down-voting this comment should read up on conditional
probabilities and Bayes's theorem. (Hint: there are only seven billion people
alive, and very few of them have their chromosomes counted.)

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maeon3
It is possible that two of his chromosomes fused to become one. Primates have
48 chromosomes and we have 46, the reason this is so is because millions of
years ago two primate chromosomes fused to become one, the proof is right
there in your DNA, we've even located the source of the recombination, where
deactivated chromosome stop codes (telomeres) exist where they don't belong.

<http://www.youtube.com/watch?v=zi8FfMBYCkk>

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dangrossman
That is what this article is about, both the explanation of his 44 chromosomes
and the relationship to other primates (including past humans) and their 48.
The article even embeds a Ken Miller YouTube video.

You should read (or at least skim!) before commenting.

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maeon3
I leapt to the right conclusion without reading the article. I'm a lazy
genius! reading through the article I realize they came to the same conclusion
I did.

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sirclueless
"Humans have 46 while chimpanzees (and many other great apes) have 48... Over
time, people with 46 chromosomes won out over people with 48."

Well guys, I think we found the future of the human race.

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hugh3
At the risk of turning into reddit:

[http://media.photobucket.com/image/recent/secondgryphon/Sket...](http://media.photobucket.com/image/recent/secondgryphon/Sketches/EVA/EVOLUTION-
DOES-NOT-WORK-THAT-WAY.jpg)

