The 44 Chromosome Man

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.

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.

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.)

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.

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

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?

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!

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

> 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.

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

"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.