38 I The Process of Evolution 



biologists who recognized their significance. Almost overnight, 

 genetics became an important and rapidly developing field of biol- 

 ogy. However, there were many scientists who felt that Mendel's 

 work had little application to evolution in populations in nature or 

 to the prevailing type of continuous variation found in wild and 

 domesticated populations of both plants and animals. It was only 

 after many years of work that the evolutionary significance of Men- 

 del's laws was established. 



THE UNITS OF HEREDITY 



The units of heredity postulated by Mendel and subsequently termed 

 genes were identified as specific regions of the chromosome; they are 

 the segment between two closest points of crossing-over. More re- 

 cent work, especially in the biochemical genetics of microorganisms, 

 has led to other definitions of the gene. For example, it is generally 

 accepted that specific genes control the formation of specific en- 

 zymes. By growing microorganisms on media of known composition, 

 it is possible to show that gene mutation results in the loss of ability 

 to carry out some cellular reaction. For example, a mutant bacterium 

 may lose the ability to synthesize a particular substance, such as 

 tryptophan. The number of mutant alleles of genes has been found 

 usually to be quite large. It is necessary to think of the gene, as re- 

 vealed by these studies, as a region of the chromosome that is muta- 

 tionally complex. Benzer has referred to the possible mutational sites 

 within a gene as mutons. Evidence suggests that a single muton may 

 consist of only one or a very few nucleotides. 



At this level of study, experiments have shown that recombination 

 may occur within the limits of a single gene, i.e., within a functional 

 unit. The smallest unit that is interchangeable is called a recon. 

 Recombination studies show that the recon also is about the size of 

 a nucleotide and that the mutons in the functional unit are arranged 

 in linear fashion. Since it appears to make no diflFerence pheno- 

 typically how the genes are arranged in an organism heterozygous 



for two factors, i.e., whether the genes are arranged ~^ or — ^, it is 



interesting to ask the same question about parts of genes. Will 

 enzyme synthesis take place just as well when the mutants within 

 one gene are distributed between the two chromosomes (the so- 

 called trans state) as when the mutants are on one chromosome (the 

 cis state)? The answer is not clear, but occasionally parts of the 

 gene may be divided between the homologous chromosomes, al- 

 though the smallest group of mutants that must be in the cis posi- 



