Equilibrium under inbreeding in balanced polymorphisms.

The results of a study on frequencies of genes causing lethal or sublethal disease are reported in relation to the level of consanguinity and heterozygote advantage in populations. The theoretical background of the population dynamics of genes under heterozygote advantage and in the presence of inbreeding is examined and the theoretical models are applied to the estimation of the level of heterosis for lethal and sublethal genes that are responsible for important public health problems in certain populations; these problems include beta-thalassaemia and sickle-cell anaemia. Estimates obtained from the proposed models show a high degree of internal consistency under widely different conditions; data on sickle-cell anaemia from several African populations and on beta-thalassaemia from Italy and Malta were analysed. The hypothesis that rare lethal genes, such as those for Tay-Sachs disease, are maintained in the human population by heterozygote advantage is examined and discussed.