Possible advantages from a fused chromosome

A genetic change can spread through a population in one of two ways. It can spread by random genetic drift or it can spread because it confers some kind of advantage to the owners. Clearly the spread of a fused chromosome mutation would be more likely to spread if it were advantageous.

The previous page discussed some of the problems that an intermediate odd number of chromosomes (such as 45) would cause. These problems mean that a fused chromosome would probably be at a selective disdvantage unless the fused chromosome also conferred an advantage which outweighed the fertility problems.

Possible advantages from a fused chromosome:

Gene linkage
Genes which were originally remotely located on two chromosomes can become linked by being brought together on a single chromosome. If two genes are on the same chromosome then there is a higher probability that they will be propagated together, the closer they are together then the lower the probability is of them being separated again during recombination.

If they were a particularly good combination then this could be advantageous. For example linking a gene for longer legs and another for more energy efficient muscles might lead to offspring who could run longer distances, thus giving them an advantage whilst hunting.

Telomere reduction
At the ends of each chromosome there are regions called telomeres which protect the gene at the ends of the chromosomes. These regions become shorter each time the cell divides and when the telomeres run out the genes they were protecting will start to be damaged. This means there is a limit on how many times cells can divide and thus on the lifetime of the owner. Telomeres are regenerated in gametes and stem cells so each generation starts with a fresh set.

When a chromosome fuses with another it loses two chromosome ends (at the join) so if there were particularly vulnerable genes at those ends then they would now have better protection, which could lead to greater longevity.

Changed phenotype
A small amount of genetic material is lost from the short arms when acrocentric chromosomes fuse. If the genes are not vital or are repeated elsewhere in the genome then this may not cause a serious problem but could still change the phenotype of the owner.

The loss of a gene might lead to a change in the expression of a protein, which in turn could be advantageous. An example where the change in the expression of protein turned out to be beneficial was the reduction in size of the jaw muscles running up the side of our heads. This is thought to have allowed a larger braincase to evolve (Ref 1).

The change does not even have to confer a direct physical advantage, for example if the fusion of the chromosomes change the physical appearance of their owners then the new "look" could be considered attractive. This would increase the reproductive success of the people with the mutation thus spreading it through the population.

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  • Runner picture from Wikimedia Commons
  • Ref 1 : Stedman, H. H. et al., Myosin gene mutation correlates with anatomical changes in the human lineage,Nature 428, P415-418