“…As a genetic recipient in the cross with an Escherichia coli K-12 Hfr strain, Salmonella typhosa is capable of recombining transferred K-12 deoxyribonucleic acid (DNA) with homologous regions of its own chromosome (1,3,4). Frequently, however, the E. coli DNA persists, unrecombined, in S. typhosa as an addition to the Salmonella genome, thus creating a partially diploid hybrid (1)(2)(3)(4). When an inherited E. coli gene determines a positive phenotypic character for which the corresponding Salmonella allele is a negative determinant, its diploid nature may be discovered by its loss, i.e., segregation, from the S. typhosa hybrid.…”