214 ERNST FREESE 



tlic best illu>tr;iti()n of this fart is Fi<i;. 3. One may thus idciitil'v the word 

 "jione" with only one concept, and best with a functional unit (even 

 that requires certain qualifications). "Gene mutations," then, ai'e muta- 

 tions that alter only one functional unit while "chromosomal mutations" 

 affect several functional units at the same time. In order to define a 

 functional unit ojierationally one can use the "cis-tmns" test by which 

 mutations that cannot cross-feed are attributed to one "cistron" (Benzer, 

 19571. For this test two homologous chromosomes, each cairying one 

 mutation (/ra/?s-position), are i)laced in a conniion cytoplasm; when this 

 combination does not show the nonnal functional activity the two 

 mutations are said to belong to the same cistron. The corresponding cis 

 test, in which both mutations are on one chromosome while the homol- 

 ogous chromosome is non-mutant, serves to establish that normal func- 

 tional activity is obtained when one chromosome is not mutated. 

 Sometimes the cis-tmns test is not sufficient to demarcate the boundaries 

 of a functional unit, for example, when cross-feeding is not possible 

 because the two homologous chromosomes are too distant. Another 

 complication is illustrated by the case in which it is not possible to decide 

 by qualitative growth tests whether the functional activity of the hetero- 

 caryon is normal or subnormal and caused by another repair mechanism, 

 i.e., "complementation" (see Demerec and Hartmann, 1959). In these 

 cases it is necessaiy to measure quantitatively the functional (usually 

 enzymatic) activity. Nevertheless, a suitably refined cis-trans test pro- 

 vides, in most cases, a practical definition of a functional unit (e.g., 

 the genetic material which determines the structure of a peptide). 



As another subdivision of mutations which is useful for detailed 

 genetic experiments, one can distinguish "large alterations" and "point 

 nmtations." A mutation is a large alteration if the corresponding nuitant 

 cannot recombine, and produce the standard type, with at least two 

 other mutants which in turn are able to recombine with each other. A 

 chromosomal mutation is a large alteration; but also a gene mutation 

 can be a large alteration, for examjilc, if a larger piece of a gene (DNA) 

 is missing. In contrast, one would like to define a point mutation as one 

 in which only one nucleotide pair has been altered (including deletion 

 or addition). Since it is impossible to prove at the presc^nt time that a 

 given mutant has undergone such a small change the following oi^cra- 

 tional definition will be used: A 'point mutation is one which does not 

 give evidence of a larger alteration in the above-mentioned crosses with 

 as many independently isolated mutants of the same phenotype as are 

 available, and usually it does revert spontaneously or by induction. 

 Obviously the precise definition of the "point" depends both on the 

 number atul kinrl of mutants ci-ossed and on the smallest distance on 



