H. L. CARSON 25 



few genes for special attention. We are being forced more and 

 more to emphasize the total genotype. 



The geneticist is continually trying to see in behind the facade 

 of the phenotype to the genotypic basis that is the hereditary 

 endowment of an individual. The genotype is truly a property 

 of an individual organism, a kind of personal information which 

 crosses the narrow hereditary bridge from one generation to the 

 next. Sexual reproduction generates diverse genotypes, and each 

 genotype confers individuality. It is because of this individuality 

 of the genotype that the suffix "type" in the word conveys an 

 erroneous idea. The geneticist is wholly unable to set up types 

 with properties of the types of the taxonomist or, in fact, to work 

 with types of any sort. Indeed, the major emphasis of this ar- 

 ticle arises from the fact that one of the most important corol- 

 laries of our modern understanding of sexual reproduction is 

 that types are absolutely foreign, in fact diametrically opposed, 

 to the whole concept. 



The term "wild type" has run into similar difficulties. It, again, 

 was originally used in an operational sense to refer to an allelic 

 condition at a single locus on a chromosome. An unjustified as- 

 sumption of uniformity from individual to individual is made 

 when the term wild type is extended to cover many loci not 

 under direct experimental observation. Thus, in some of the ear- 

 lier studies of genetics there was a tendency for natural popula- 

 tions of organisms to be interpreted as if the individuals com- 

 prising them were essentially homozygous for "wild-type" genes, 

 with only a few scattered recessive mutant alleles here and there 

 at various loci. This "classical" hypothesis is now giving way to 

 what Dobzhansky (1955) refers to as the "balance" hypothesis. 

 This latter designation refers to the fact that recent studies of 

 Mendelian populations under natural conditions have revealed 

 that a fantastic amount of genetic variability is concealed in 

 recessive form in the individuals of these populations. 



The mechanisms which maintain a high level of genie variabil- 

 ity in natural populations are only just now becoming clear. It 

 is not within the scope of this short paper to review them, and 



