 What about inbreeding? In general, inbreeding will tend to increase the homozygosity of a population, like positive assortment (Hartl 2020, ch. 3). This is because relatives tend to be genotypically similar. Inbreeding often has negative effects on organismic fitness—a phenomenon known as ‘inbreeding depression’. The explanation for this is that deleterious alleles often tend to be recessive, so have no phenotypic effect when found in heterozygotes. Inbreeding reduces the proportion of heterozygotes, making recessive alleles more likely to be found in homozygotes where their negative phenotypic effects become apparent. The converse phenomenon—‘hybrid vigour’ resulting from outbreeding—is widely utilised by animal and plant breeders.