Has this already happened in the human lineage?

An ancestral fused chromosome due to a Robertsonian translation will have certain features which would allow it to be identified:
  • You would expect that the original separate chromosomes would still exist in related species
  • The fused chromosome may have remains of a second centromere and/or telomeres near the fusion point
As it turns out, if we look at human chromosomes we find that chromosome 2 has both of these features. The related species are the other great apes such as chimpanzees, they still have two pairs of chromosomes where we have only one. This means they have 48 chromosomes in total rather than 46. The fusion point is quite clear if you compare the chimpanzee 2p and 2q (sometimes referred to as 12 and 13) chromosomes with the human chromosome 2 and line up the matching sequences like this:

The black bars represents a stain which bind to particular nucleotides in the sequences, for example the stain Giemsa binds to areas that are rich in adenine and thymine. The resulting banding pattern will be very similar for chromosomes which contain closely matching sequences.

As you can see the bars on the single human chromosome at the top match with the pair of chimpanzee chromosomes shown underneath.

You can also see that one of the chimp centromeres does not have a matching centromere on the human chromosome. However when that area is looked at closely the remnants of both the original centromeres (Ref 1) and telomeres (Ref 2.) have been found. The second centromere appears to have become disabled over time.

So Robertsonian translocation gives a mechanism by which chromosomes can fuse and scientists have found a chromosome in the human genome which shows all the features that we would expect a fused chromosome to show. In addition our closest relatives, the other great apes, still have homologues of the original pair of chromosomes which make up the human chromosome 2.

This is very strong evidence that at some point in our past, our ancestors had separate chromosomes which fused to become the chromosome 2 that we see today. The other great apes still retain the original two separate chromosomes.

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  • The chimpanzee and human chromosome images were sourced from Indiana University
  • Ref 1 : LaDeana W. Hillier1 et al., Generation and annotation of the DNA sequences of human chromosomes 2 and 4, Nature 434, P729
  • Ref 2: J. W. IJDO et al., Origin of human chromosome 2 Proc Natl Acad Sci U.S.A 88, P9051-9055