Compound heterozygotes are individuals carrying two different mutant alleles of the same gene. In the absence of a dominant (wild-type [wt]) allele, genetic interactions between recessive alleles (referred to here as “biallelic” effects) could result in different phenotypic outcomes including interallelic complementation. Although amelioration of disease symptoms by interallelic complementation would create an ascertainment bias in the clinic, the lack of evidence concerning interallelic complementation or other biallelic effects in human disease is likely caused by the difficulty in distinguishing such effects from environment and genetic background.