Drop of Prevalence after Population Expansion: A lower prevalence for recessive disorders in a random mating population is a transient phenomenon during and after a growth phase

Despite increasing data from population-wide sequencing studies, the risk for recessive disorders in consanguineous partnerships is still heavily debated. An important aspect that has not sufficiently been investigated theoretically, is the influence of inbreeding on mutation load and incidence rates when the population sizes change. We therefore developed a model to study these dynamics for a wide range of growth and mating conditions. In the phase of population expansion and shortly afterwards, our simulations show that there is a drop of diseased individuals at the expense of an increasing mutation load for random mating, while both parameters remain almost constant in highly consanguineous partnerships. This explains the empirical observation in present times that a high degree of consanguinity is associated with an increased risk of autosomal recessive disorders. However, it also states that the higher frequency of severe recessive disorders with developmental delay in inbred populations is a transient phenomenon before a mutation-selection balance is reached again. Author summaryWhat determines the recessive disease burden? The empirical observation that the proportion of intellectual disability of autosomal recessive cause is usually higher in offspring of consanguineous partnerships may lead to a misunderstanding about the mechanisms at work. In any population, selection removes pathogenic alleles from the gene pool while the de novo mutation rate adds novel pathogenic alleles. For comparable mutation rates, the incidence of severe recessive disease should be comparable in mutation-selection balance, regardless of the mating scheme. Different incidences can therefore only be explained by population dynamics that are not in equilibrium. We studied in simulated populations the time scales in which mutation-selection balance is reached after a growth phase and found that the mating scheme has a big impact on this lag time. When cousins mate preferentially with cousins, a few generations after a ten-fold increase in size the new equilibrium is established. In contrast, for random-mating, the transient advantage of a lower incidence may last for hundreds of generations, while the mutation load increases. By this means, our findings also highlight the importance of better carrier screens in the future for genetic consultations.