The observed biallelic effects were of three general types. In the first, the allele associated in a homozygous state with a phenotype closer to wt singularly determined the phenotypic outcome, a phenomenon widely known in human recessive disease. Because these Xpd alleles functioned at or near wt levels with respect to a particular function, we call these effects “dominant”. Such alleles can also be referred to as “separation of function” alleles, because they allow dissection of the roles of multifunctional proteins in specific phenotypes.