Tag Archives: genetics

The Great Chromosome 2 Fusion Event

In a slight break from what I usually write about, someone recently asked me a question about the evidence for common ancestry between humans and the 3 other great apes, Chimpanzees, Gorillas, Orangutan, that make up the Hominid taxonomic family. Now I don’t often get to talk about topics that I dealt with in my previous life as a researcher in biology so this was a bit of a fun diversion for me. I’m always slightly taken aback when someone professes ignorance for one of the greatest and well supported scientific theories of modern science, especially given the richness of information available to the internet generation. Then again not everyone has had an education where evidence held primacy or possibly has the interest (or in my case fascination!) with all things biological.

One of the greatest and most overt pieces of evidence for this is shown directly in our chromosomes and in the face that we have 46 of them while the other 3 hominids have 48. Now we know that removing an entire pair of chromosome is uniformly lethal. There are no cases where a hominid can lose this much genetic information since each chromosome has too much information in it that is required to create a viable organism. So where did it go?

Nowhere. It’s right there hiding in plain sight. If you lined up the 24 chromosomes of a chimpanzee and the 23 chromosomes of a human you would notice 2 things: firstly, and most obviously, you would have a pair of chimpanzee chromosomes left over and secondly, you would see that the human chromosome 2 is much bigger than the chimp on. You would also notice that if you stuck the leftover chromosome alongside chromosome 2 it matches almost perfectly. This is because each individual human chromosome 2 is actually 2 ancestral chromosomes stuck together, which explains where all the genetic information went.

We can see this even more obviously by 3 facts about the chromosomes:

  1. The genetic sequences between human and chimpanzees are almost identical and code for almost the identical genes
  2. There is an vestigial centromere in the middle of the long arm of the chromosome
  3. There are 2 vestigial telomeres located between the functioning centromere and the vestigial one above.

The the first point, we would not expect the sequences to be completely identical since there has been several million years of separation between us and the other great apes. If you want to look further into this, the topic of nested ERV’s is particularly interesting. For the second and third, what we see are the structural remnants of the fact that the human chromosome 2 previously existed as two separate chromosomes. A centromere is the bit of the chromosome where the 2 arms cross and is very important during cell division when it helps the chromosomes separate. The telomere is kind of like the aglet on a shoelace and is responsible for making sure the chromosome does not unwind or fray.

Here’ an awesome illustration of this.

So this is overwhelming evidence that this is where the “missing” chromosome went but doesn’t this fly in the face of probability? We know that Robertson translocations like this can often be either lethal or lead to fertility. Likewise, the first time this happened the first individual with 23 chromosome would have no similar individual to mate with. If they mated with the previous 24 type, surely there would be a mismatch between the chromosomes that would lead to non-viable or infertile offspring as seen in horses and donkeys? Wouldn’t this risky change have to happen twice?

Well the evidence demonstrates otherwise. We have seen chromosome fusion actually happening in cows [1] so we know that this sort of mutation is not always lethal. We also have evidence that demonstrates that chromosomal heterozygosity does not prevent the individual from mating successfully with member of their species with the normal chromosomal arrangement [2]. These 2 observations demonstrate that it not only is it not statistically improbable for this to happen but well supported as a pathway and remains as one of the neatest and most easily understood examples of evidence for common ancestry between us and the other great apes.

[1] A new centric fusion translocation in cattle: rob (13;19).Molteni L et al. Hereditas. (1998)

[2] Chromosomal heterozygosity and fertility in house mice (Mus musculus domesticus) from Northern Italy.Hauffe HC et al. Genetics.1998