RECOMBINATION ANALYSIS IN MICROBIAL SYSTEMS 45 



rise to complete virus, and those that do not are not recovered. Thus 

 it is impossible to isolate the products of a single mating event. On the 

 other hand, phage systems offer great advantages. One is that the 

 crosses can be performed and progeny scored with great rapidity. More 

 important is the fact that the phage chromosome is very short, com- 

 pared to the chromosomes of higher organisms, and it therefore con- 

 tains fewer genes and fewer recombination units than any other 

 hybridizing system with an equal biparental contribution. Its length 

 in terms of nucleotide sequence is established (Thomas, 1959). 



Fine structure of the gene 



When a chromosome is examined in the detail that these systems 

 permit, new features of its structure and behavior become apparent. 

 The validity of considering the gene as an ultimate corpuscle had al- 

 ready been questioned in the experiments of Dubinin (1929), Sere- 

 brovsky (1930), and Lewis (1945), working with Drosophila, and of 

 Stadler ( 1954), working with maize. With the high-powered analysis in 

 the systems just described, the gene could be seen to be divisible into 

 numerous subunits. If one selects, in Aspergillus or Neurospora, for ex- 

 ample, a number of independent mutations having identical pheno- 

 types and affecting a single character, crosses between pairs of mutants 

 will regularly give rise to rare recombinant cliromosomes which are 

 normal with respect to the character in question. In many instances the 

 character involved has been shown to be the synthesis of a single 

 enzyme, and there is therefore no doubt that the different mutants are 

 allelic forms of a single gene locus. Nonetheless, recombination can 

 occur between them, producing a non-mutant (wild-type) chromo- 

 some on the one hand, and a doubly mutant chromosome on the other, 

 as a result of recombination. 



With most crosses between non-identical alleles, such recombina- 

 tion is rare, and if one were to set about to find it either by dissecting 

 asci and determining the nature of each meiotic chromatid, or by the 

 analysis of randomly selected spores from a given cross, one would suc- 

 ceed only by analyzing 100,000 to several millions of spores. A case in 

 point is described by Calef (1957) for two adenineless alleles in As- 

 pergillus. The varieties of recombinant chromosomes found by plating 

 406 spores and determining the genetic constitutions of the chromo- 

 somes of the spores in crosses, shown in Figure 2, are indicated as 

 "unselected recombinants." In none has a cross-over occurred between 

 adi5 and adi7. If, now, instead of choosing spores at random, one makes 

 a suspension of a large number of spores and plates them on a medium 

 lacking adenine, the only spores that will grow will be those having a 

 genetic constitution enabling them to synthesize adenine. These could 



