Inbreeding is usually deleterious for three rather different reasons.Â
1. Recessive alleles Alleles can be dominant or recessive to one another, but recessive alleles remain unexpressed in heterozygotes that also carry a dominant, i.e. recessives are expressed in females only when homozygous. They can be expressed in every normal hemizygous drone, in whom the concepts of recessivity and dominance are inapplicable, although for reasons of their function, expression of some genes is restricted to one or other sex.Â
 2. Sex determination In bees, development of female anatomy and physiology is triggered in the larva by heterozygosity at a specific locus carrying the sex determining gene, i.e. by the presence of two different sex determining alleles. By contrast, an embryo develops as male when only one kind of sex determining allele is present. This occurs normally in unfertilised eggs, described as "haploid", meaning having only one set of chromosomes. It can however also arise in fertilised eggs (described as "diploid", meaning that they carry two sets of chromosomes) if they are homozygous at the sex gene locus.Â
 3. Polygenic conditions Quantitative characters, such as honey yield, tend to be polygenic, i.e. controlled by the combined effects of genes at several loci. Outbreeding gives rise to new gene combinations, simultaneously creating heterozygosity for many genes and offering scope for improvement with respect to polygenic, complex characters such as foraging capacity and disease resistance. This is the basis of "hybrid vigour", or "heterosis".