By this definition, it is alleles, not genes, that are observed to be the units of segregation. Allelism is also conferred from the shared properties of similar mutant phenotypes and the failure to complement other mutant alleles. Alleles will almost invari ably segregate from each other in trans heterozygotes, at least in multicellular organisms having low levels of recombination. Rarely is a wild-type recombinant progeny observed to indicate that those parental alleles cannot be mutant in the same loca tion. Instead, they are described as ‘pseudoalleles’, at different sites but still regarded as marking the same gene. Other exam ples pose more of a challenge to the idea of one gene, however. In the phage T4, mutants of the rII class, named after their phenotype, map to one section of the linkage group in a cluster that is over seven map units long! Do rII mutants mark one gene? Employing what he called the cis–trans test, Seymour Benzer described the mutants in the cluster as forming two complementation groups, or cistrons. He showed that mutants in one cistron localized by recombination studies to one side of the cluster and mutants of the other cistron localized to the other side. Moreover, Benzer showed that the T4 linkage group is linear both inside and outside the rII cluster. Mutation posi tions are continuously distributed along this line with no obvious demarcations between the flanking adjacent genes or the rII cistrons. This means that genes cannot be separated solely by mutant position or by mutant phenotype. The remaining alternative, defining a gene by complementation testing, implies that genes can be separated on the basis of biochemical function.