172 CELL HEREDITY 



TABLE 6.4 



Smallest Distances Measured in Seven Organisms, 



Expressed as Fractions of the Total DNA 



(After Pontcforvo, 1958, Trends in Ccnclir Analysis. 

 New York: ('oliimhia I'niversitv Press, p. 25) 



Organism Total 'I'otal Nucleo- Minimum Minimum Minimum 



Linkage DNA tide Distanee Distance Distance 



Map in Pairs Measured Measured Measured 



Units Nucleo- per in as in 



tide Map Map Fraction Nucleo- 



Pairs I'nit Units of Total tide 



Map Pairs 



Phage T4 800 8 x 10' 1 x 10" 2 x 10"" 2.5 x lO"'^ 2 



E.coli 2000 1 x 10' 5 X 10* 2 x 10"^ 1 x 10"*^ 10 



Asper- 

 gillus 660 4 x 10' 7 X 10' 1 X 10"' 1.5 x 10"^ 7 



Nettro- 



spura 302 4 x 10' 1 x lO'^ 1 x 10"^ 3 x 10"*^ 100 



Droso- 



philu 280 8x10' 3x10-^ 8x10"' 2.9x10"*' 24 



Mouse 1954 5 x lO'' 3 x lO" 2x10"' 1x10"' 600.000 



Corn 1350 7x10^ 8x10'' 2x10"' 1.5x10"' 1,600,000 



subunits. The word "allele" may be used to refer to alternate state.s of 

 the whole gene, or it may describe alternate states in some smaller unit. 

 For example, a muton consisting of the base pair A:T may have T.A, 

 G.C, and C:G as allelic states. The presence of other bases or of other 

 chemical configurations at the site would yield more possibilities, and 

 deletion or duplication of the site might also be recognized as additional 

 allelic states. 



The fine structure of genes has been revealed in all organisms where 

 it has been looked for; apparently it is a general situation. The allo- 

 genic transformations of capsule type in pneumococcus (Figure 5.12) 

 and the transformations yielding double capsular types and intermediate 

 levels of sulfonamide resistance are probably achieved through recom- 

 bination within a functional unit. Genes concerned with the fermenta- 

 tion of lactose have been studied in E. coli heterozygotes. One, respon- 

 sible for the permease that pumps lactose into the cell, is divisible into 

 five mutable sites which, when mutant, revert at different rates. There 

 are also closely linked cistrons concerned with the synthesis of the 

 enzyme, lactase, and with the inducible versus constitutive nature of 

 this enzyme. A series of linked cistrons affecting the utilization of 



