70 The Source of Variability 



In an operational sense, the gene can be defined only as the 

 smallest segment of the chromosome that can be shown to be 

 associated consistently with the occurrence of a specific genetic 

 effect. As Stadler pointed out, such demonstrations are a function 

 of the limitations of whatever experimental methods are employed; 

 improved or new methods could conceivably demonstrate an "op- 

 erational gene" quite different from that recognized now. 



Unquestionably there are differences in some structure of the 

 chromosome at the molecular level which distinguish one gene 

 from another. Ideas as to what these differences are constitute 

 the hypothetical gene. 



The operational gene, therefore, is based on what a gene does, 

 a hypothetical gene on what it might be. Except for parts of the 

 discussion on gene change later in this chapter, I am using the 

 term gene in the sense of Stadler's operational gene. 



Cytoplasmic Inheritance 



Information concerning cytoplasmic inheritance is meagre at best, 

 but what is known about its occurrence and action is highly 

 intriguing. Examples of this type of inheritance in plants include 

 series of genes influencing pigmentation of the plastids and other 

 series of genes influencing resistance to or dependence on certain 

 chemical compounds such as streptomycin. Some of these cyto- 

 plasmic elements appear to be under the ultimate control of chro- 

 mosomal genes (Darlington, 1958), but in other cases there is no 

 evidence of such a relationship. Sager (1960) pointed out that in 

 the alga Chlamijdomonas certain cytoplasmic elements controlling 

 streptomycin resistance could combine with chromosomal genes 

 also controlling streptomycin resistance and produce an additive 

 effect. From this circumstance she concluded that the cytoplasmic 

 elements fitted the terms gene and allele as applied to chromosomal 

 elements which behave similarly. 



In mites of the genus Tetranychus, Boudreaux (1959) discov- 

 ered a cytoplasmic element causing a reduction in the number of 

 hairs on the legs (Fig. 26). This element could be destroyed by 

 heat, hence Boudreaux drew attention to its virus-like quality. 

 This fact of heat-intolerance, however, does not necessarily militate 

 against the view that the element is a true allele. 



In this regard certain observations on viruses are pertinent 

 (Hayes and Clowes, 1960). Certain phage viruses apparently can 

 exist either as a cytoplasmic inclusion or attached to the nucleus 



