[nudephil (OP)]

This week's question comes from listener Mattie:

If humans have too many or too few chromosomes it can cause them to be infertile. So how did different creatures get different numbers of them? For example, humans have 46 chromosomes, but mice have 40.

Can anyone assist?

[evan_au]

If humans have too many or too few chromosomes it can cause them to be infertile.

Having too many copies of a gene (or not enough active copies) is a common cause of genetic disease.

Having unpaired genes mostly results in infertility, since the children would have too many or too few copies of genes, and also have unpaired chromosomes.
- We see this with hybrids like a mule (63 chromosomes) = Horse (64 chromosomes) and Donkey (62 chromosomes)

In plants, doubling the number of chromosomes (polyploids) often yields a viable plant (but one which is not fertile with its parent species). But this does not work for animals.

There is a way that you can retain the same number of genes while changing the number of chromosomes: Join two chromosomes together, (or potentially, split one chromosome in two).
- An individual with this mutation will be infertile with the parent population
- But the children of an individual with such a fused chromosome will be partially fertile with each other, and could produce a fertile population.
- This requires a bit of mutant incest.

Just such a chromosome fusion event appears to separate humans from chimpanzees.
- Human chromosome 2 appears to be the result of fusion of chimpanzee chromosomes 2A and 2B.
- This gives humans 23 chromosome pairs, compared to 24 pairs for most ape species.
- It is thought that Neanderthals & Denisovans also had 23 chromosomes, since humans were fertile with these other hominids
- But it's hard to be sure from fossils, because ancient DNA is broken up into small pieces.
See: https://en.wikipedia.org/wiki/Chromosome_2#Evolution

See: https://en.wikipedia.org/wiki/List_of_organisms_by_chromosome_count

[EvaH]

Asked and answered on this week's show!

Listen in here https://www.thenakedscientists.com/podcasts/question-week/how-did-varying-chromosome-numbers-evolve