IBD vs IBS

Question

[MAPK]

I was reading these two papers, (http://www.nature.com/nrg/journal/v11/n11/full/nrg2865.html and http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3025522/) where I got confused with the way they have discussed IBD and IBS and the way formula for IBD was derrived in Powell et al. What I have understood is IBD is the measure of descent between two individuals, whereas IBS is just the allelic similarity between to individuals at the given loci regardless of their common ancestry. Can someone please explain me how to calculate IBD and then estimate IBS from it (or is it vice versa?) in simple terms?

Answer 1

[Sean Davis]

All alleles that are identical by descent are identical by state, as well. Alleles that are identical by state may or may not be identical by descent. One calculate identity by state without information about inheritance while identity by descent requires some estimate of the likelihood of inheriting the SAME copy of an allele.

[Shicheng Guo]

Quite clear answer! might be clear.

[MAPK]

Thanks Sean. So the IBS doesn't infer any relationship state between the pair of individuals? For example, with IBS=0.5 can we say that the pair of individuals are first degree relatives? How does IBS=0.5 differ from IBD =0.5 in practical?