Genes-. Nature and Mode of Action - 529 



genie segment in both of the replicating 

 DNA strands (Fig. 27-4). In this case, neither 

 gene can function by itself. But if the two 

 strands are replicating simultaneously in the 

 same bacterial cell, an exchange of parts 

 within the limits of the single gene may give 

 rise to one perfect unit (Fig 27-4). This can 

 only occur, however, // there is no overlap- 

 ping of the deficient sections, as is shown dia- 

 grammatically in Figure 27-4. 



An analogous phenomenon has been re- 

 ported by Green and Green, studying the in- 

 heritance of three mutational changes in 

 Drosophila. These mutations appear to be 

 localized very near each other in the chromo- 

 some, probably at different points along the 

 length of the same gene. All three mutations 

 have a similar effect upon the shape of the 

 eye. Each produces what is termed a "lozenge 

 eye" when it is homozygous. Consequently, 

 these mutations have been designated as Iz 1 , 

 Iz*, and k». 



The curious thing about the inheritance 

 of the Iz mutations is that they reach expres- 

 sion, even when heterozygous, if they are 

 localized oppositely (trans-position) in the 

 pair of chromosomes — as is shown in Figure 

 27-5. When the loci are in the same chromo- 

 some (cis-position) the mutant genes are in- 

 active. One perfect (nonmutated) genie unit 

 seems necessary for the production of a nor- 

 mally formed eye, just as a complete unit is 

 necessary for function in T 4 bacteriophage. 

 Some recent workers prefer to call such an 

 integrated series of mutational sites a cistron, 

 rather than a gene, but this appears to be 

 mainly a matter of semantics. 



ROLE OF THE GENES IN DEVELOPMENT: 

 THE ONE GENE, ONE ENZYME CONCEPT 



Even before it was known that DNA can 

 determine the synthesis of enzymes and other 

 proteins, considerable evidence indicated 

 that genie defects were correlated with en- 

 zyme deficiencies. Several examples of such a 

 relationship have already been cited; for ex- 

 ample, the case of phenylketonuria, in which 



SEGMENTS IN A PAIR OF CHROMOSOMES 

 NORMAL EYE LOZENGE EYE 



C/S-POSITIONS 



TRANS-POSITIONS 



Fig. 27-5. The effects of a series of mutational 

 changes may depend upon their relative positions in a 

 pair of chromosomes. The development of a normal 

 eye depends upon the existence of one normal gene 

 (or gene series) in at least one of the chromosomes. 

 LZ 1 , LZ 2 , and LZ 3 are nonmutated sites, which in series 

 yield an eye of normal shape; Iz 1 , Iz 2 , and /z 3 are 

 mutant sites, which yield an abnormal (lozenge- 

 shaped) eye if they interrupt a series of nonmutated 

 sites. 



a defective gene determines the absence of 

 one enzyme, and of alkaptonuria, in which 

 another deleterious mutation is associated 

 with another missing enzyme (Chap. 26). But 

 final demonstration that such a relationship 

 frequently (if not always) obtains awaited 

 the ingenious experimental work of Beadle 

 and Tatum. 



Beadle and Tatum studied the inheritance 

 of mutational changes induced by x-ray irra- 

 diation in the red baker's mold, Neurospora 

 crassa. This mold provides an exceedingly 

 favorable material. It has a short (10-day) life 

 cycle and the mycelium that grows from each 

 spore is haploid. Thus any recessive mutation 

 is not obscured by a dominant allele; that is, 

 the effect can be observed in the first genera- 

 tion. Moreover, the hereditary transmission 



