Past exam of the mathematics course of the University of Cambridge / 2006 / iii / Paper 45 / 3 / a

Fix one particular ancestral allele copy $q$ among the four copies in the founding couple. The pedigree founder carrying it transmits it independently to each of the two generation-2 siblings with probability $1/2$. ThereforeHere “one copy” means a specified copy, as required to follow “this ancestral copy” in the subsequent parts. If instead one asks whether the siblings share at least one of the four copies without specifying which, the probability is $1-(1/2{)}^2=3/4$: they fail to share either the paternal or maternal copy with probability $1/4$. These are different events.