Genetics 39 



tion is rather large. These examples of complementation between 

 mutants are somewhat equivocal, however. The cis-trans test may 

 be used to specify the functional gene units, which have been called 

 cistrons. 



In the study of populations, the unit of heredity must be given a 

 strictly operational meaning within the context of the study, as, in 

 fact, it must be in all biology. In evolutionary studies this unit can- 

 not be the same as that in biochemical genetics. This must be kept 

 clearly in mind when discussions of genes in an evolutionary con- 

 text are compared with those based upon studies of microorgan- 

 isms in the laboratory. In studying inheritance in populations in 

 nature, the unit of heredity in most cases becomes a statistical one, 

 for the factors controlling the expression of continuously varying 

 traits are numerous and complexly interrelated. At present, only the 

 methods of the statistician can sort out the interactions of the he- 

 redity units (which are assumed to be similar to those affecting dis- 

 continuous variation), the developmental systems through which 

 they are expressed, and the effects of the environment on both these 

 systems. The environment of an organism at any particular time or 

 place is unique and not repeated or repeatable. This means that in 

 experimental studies it is important to make replicate experiments in 

 space and time— an unfortunately expensive and time-consuming 

 process. In the following pages the basic facts of the inheritance of 

 discontinuous and continuous variation will be summarized, to- 

 gether with a discussion of those aspects of gene beha\ior that are 

 particularly important to evolutionary studies. 



MENDEL'S LAWS 



The basic rules of heredity deduced by Mendel are familiar to any- 

 one who has had an elementary course in biology. In crossing peas, 

 Mendel found that, when differing parents were mated, the first 

 generation offspring (Fi) resembled one or the other parent. The 

 trait expressed in the Fi he referred to as dominant to that which 

 did not appear (the recessive). Crossing the essentially uniform F, 

 plants to produce a variable F2 generation in which individuals with 

 the recessive trait appeared, showed that the factors responsible 

 for the appearance of the traits were not lost but merely hidden. By 

 a study of the types of progeny in the Fi and Fo, Mendel deduced 

 that each offspring contains two homologous factors, one received 

 from each parent, affecting the expression of the traits studied. .\n 

 Fi offspring from a cross between differing parents must contain two 

 different but homologous factors, one for the dominant trait and 



