Effect of α+- thalassemia frequency on the relative fitness of inbreeding populations in the stochastic (upper panel) and analytic model (lower panel). In the upper panel, initial negative excess of relative fitness (blue bars) in the inbred population is the effect of recessive lethal alleles. An excess of α+-thalassemia allele (red bars) is seen after 5–6 generations – after allele frequency (black S-shape line) is increased to around 0.2. Relative excess of α+-thalassemia is maximal when its frequency is in the middle of the range (~0.35–0.7). Results are for n = 1000 and the ratio of differential survival = 1.39. In the lower panel, the calculated relative fitness includes only the effect of α+-thalassemia. The fitness ratio is the size of inbred population divided with that of outbred population. The results are for n → ∞ and the ratio of differential survival = 1.39.