Saturday, 18 October 2014

Identity by descent: a confusing concept

There's a lot of discussion of kin selection using the term "identical by descent" and "identical by state". The idea of "identical by descent" is that genes are shared as a result of direct descent from a common ancestor - without recombination or mutation. "Identical by state" just means that the DNA sequence is shared. It is said that 50% of their genes with their daughters "IBD".

I think the "identical by descent" terminology is confusing and not useful. In biology, if genes are identical, they are practically always identical through being copied from one (or more) shared ancestors. Mothers share more than 50% of their genes with their daughters - due to genes that have reached fixation, inbreeding and so on. However they still share these gene sequences due to descent from shared ancestors. As to a gene mutating into another form and then mutating back again. If you do the sums, for a gene of any reasonable size this rapidly becomes ridiculously unlikely. There's too much scope for neutral mutations elsewhere. In practice, when genes are identical, the odds are enormously in favour of this being due to shared ancestry. The idea that recombination with an identical gene makes genes not "identical by descent" is an awful one. They are still "identical by descent" - just descent from various different ancestors.

You can't say that mothers share 50% of their genes with their daughters "IBD". It is confusing and mistaken. If you want to use the 50% figure, you have to find another reason for doing so.

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