Past exam of the mathematics course of the University of Cambridge / 2006 / iii / Paper 45 / 2 / d / Solution

Summing the transmission table by its rows and columns givesThus the marginal case-versus-pseudo-control odds ratio isThis is the requested approximate allelic effect. Keeping the parent-specific matching instead gives the discordant-pair odds ratio $c/b=30/20=1.5$, numerically close here. Neither calculation makes the within-family allele observations independent population samples.