BACTERIOPHAGE GENETICS 353 



the nucleic acid segment might be anything from an exchange 

 of one nucleotide for another to a deletion of the entire functional 

 unit. It is clear that the mutational unit may be much smaller 

 than the functional unit and that many different mutations may 

 occur at different loci within one functional gene unit. If one 

 can determine which mutant loci fall within the functional unit, 

 one can determine the extent of the functional unit in terms of 

 recombination frequency. If one depends on phenotype alone 

 this may be a difficult problem because r mutations, for instance, 

 occur in a number of functional units. A test for "pseudo- 

 allelism" developed in studies with higher organisms is used as 

 a definition of the functional unit. To decide if two mutants 

 with the same phenotype involve the same functional unit, they 

 are tested in diploid heterozygotes containing the two mutations 

 in different configurations. If the heterozygote contains both 

 mutant loci in the same chromosome (cis configuration) the 

 phenotype is usually "wild" because the second chromosome pro- 

 vides a complete set of functional units. However, if the heter- 

 ozygote contains one mutant locus in each chromosome (trans 

 configuration) the phenotype depends on whether one or two 

 functional units are involved. 



If the two mutations involve different functional units, the 

 trans heterozygote will be wild type phenotypically because an 

 intact functional unit corresponding to each mutation will be 

 present. If the two mutations have occurred in the same func- 

 tional unit, the trans heterozygote will hav^e the mutant pheno- 

 type because neither chromosome will have a normal functional 

 unit and so neither chromosome can determine the synthesis of a 

 normal protein molecule. Therefore the results of the cis-trans 

 test can be used to determine whether two mutations have 

 occurred in the same functional unit or not. If a number of 

 spatially nonidentical mutations in the same functional unit 

 (pseudoalleles) can be mapped, the size of the functional unit 

 can be determined. In phage work mixed infection of single 

 cells with two genetically distinct phage particles simulates a 

 diploid heterozygote because both genetic constitutions are func- 



