Co-recessive inheritance: a model for DNA repair and other surveillance genes in higher eukaryotes.

The co-recessive inheritance hypothesis proposes that certain recessively inherited diseases require homozygosity and/or hemizygosity for defective alleles at more than one locus simultaneously for the trait to be expressed. Although this hypothesis was originally proposed in the context of defective alleles for genes coding for DNA-repair functions, it need not be limited to this context, and genetic selection pressure may favor this model for genes involved in surveillance of any type. The co-recessive inheritance hypothesis also predicts extremely high carrier frequencies, likely affecting much of the general population, for defective alleles associated with these rare recessive diseases. The model predicts much lower rates of consanguinity between the parents of affected individuals than autosomal recessive inheritance, allowing it to be tested epidemiologically, and recent data suggest that the hypothesis may be valid for some cases of ataxia telangiectasia and xeroderma pigmentosum. The model provides possible explanations for a number of otherwise puzzling findings in several diseases associated with defective DNA repair.