Ancestral inference. II. The founders of Tristan da Cunha

The Tristan da Cunha data for the MN-blood types and for retinitis pigmentosa are analysed according to the methods for ancestral inference developed in the preceding paper. Joint and marginal likelihoods for the MN data show that it is possible to make inferences about the types of original founder genes, although relative values are in some cases not large. Although relative likelihoods are robust against small changes in gene frequency, they can be distorted by assuming values that differ widely from the maximum likelihood estimate. In the case of retinitis pigmentosa, under an assumed allele frequency of 0.01, we conclude, contrary to expectations, that the couple (975, 798) are substantially the most likely founders to have been responsible for the introduction of the allele. The inferences for this trait are complicated by the fact that it is recessive, and genotypes are therefore not observable. Extinction probabilities for a variety of sets of original founder genes on the Tristan da Cunha ancestral pedigree are also computed and discussed.