166 CELL HEREDITY 



, mixrcl intcctioii uixcs the mutant phcnotNoc. and \('r\ lew cells 



-1-104 ' ' 



Ivsf. It till' two mutants hclont:; to (lillcrcnt scynK'nts, .A .md H. e.g., 

 , extensive Ksis occurs, vicldint: both tvpcs ot nliaiics and recom- 



binants. Therefore, the two segments of the rll region correspond to 

 different functional units hut the units within each segment behaxe 

 as pseudoalleles. 



It is clear that genes as functional units are large and contain many 

 smaller mutable units separable by recombination. An estimate can be 

 made of the size of these units. The smallest recombinable unit, called 

 a recon, is no larger than the smallest observed nonzero recombination 

 value; it is around 0.02 map units in length. Similarlv, the size of the 

 mtiton can be estimated if it is defined as the smallest element, altera- 

 tion of which can be effective in causing a mutation. One method in- 

 volves observing the number of mutational changes that can exist within 

 a given map length. Nine separable changes were located within a 

 distance of 0.8 map units setting an upper limit of 0.09 map units for the 

 muton. Another method consists of measuring the distance between the 

 sites of two peripheral mutational alterations and comparing it with 

 the sum of the distances between each of these and an altered site in 

 between them, e.g., 



\\'ith this method it was show n that the muton must be smaller than 0.05 

 map units. 



Benzcr, who is responsible for this line of analvsis, has attempted to 

 translate these abstract genetic map constructions into a phvsieal map 

 measured in terms of the number of nucleotide pairs in a DNA double 

 heli.x. The DNA content of phage T4 is ca. 4 x 10^ nucleotides. From 

 the star experiments (Figure 5.2) it seems that 40 per cent, or 1.6 x 10^ 

 nucleotides make up the genetic material." If there is just one copy of 

 genetic information and it has the paired structure ot the model ot 

 Watson and Oick, the total length ot the hereditarx material should be 



"But see footnote on p. 1 17. 



