CHAPTER 1 



whose members carry the same genotype, a 

 pure line. The expression of the genotype in 

 traits or characters (bean size in our ex- 

 ample) is called the phenotype. So environ- 

 mental differences have caused the same 

 genotype to produce a variety of phenotypes, 

 and we conclude that the differences between 

 the seeds of a pure line are environmentally 

 produced and are not due to any differences 

 in genotype. 



But consider next another particular bean 

 seed, of this same species of bean, that gives 

 rise to offspring beans (Figure 1-1 B) which 

 are very large, large, and medium sized. 

 Since each of these produces offspring beans 

 which again show the same range of pheno- 

 types we are clearly dealing with another and 

 different pure line, within which phenotypic 

 variability is attributable to environmental 

 fluctuation. 



How can we explain the differences be- 

 tween these two different pure lines, one of 

 which can produce very small and small bean 

 seeds while the other can produce very large 

 and large ones? All the beans were grown 

 under the same environmental conditions, so 

 these phenotypic differences cannot be due to 

 environmental differences; instead they must 

 be due to genotypic differences. So we must 

 conclude that the genetic material in these 

 two pure lines is different. 



How can we explain the fact that some of 

 the seeds in both of these genotypically dif- 

 ferent pure lines are similar — medium 

 sized? In this case different genotypes have 

 produced the same phenotype through the 

 action of the environment. 



What is the consequence of the fact al- 

 ready mentioned that under similar environ- 

 mental conditions the average size of the 

 beans produced within a pure Hne remains 

 the same regardless of the size of the specific 

 beans planted? Thus in the pure line first 

 described the offspring bean seeds have the 

 same average size whether the very small or 

 the medium seed is used as parent. Similarly 



the average size of seed produced in the 

 second pure line is the same when either the 

 medium or the very large seed is the parent. 

 In other words selection for bean size within 

 pure lines is futile, as would be expected on 

 our hypothesis that all members of a pure 

 line are genetically identical. 



Throughout the bean experiments de- 

 scribed, effort was made to keep the envi- 

 ronment the same. This does not mean that 

 the environment did not vary, but that it 

 varied approximately in the same directions 

 and to the same degree for all individuals in 

 the study. In this particular work it hap- 

 pened that phenotypic variability due to the 

 fluctuations of environment was not so great 

 as to completely mask the phenotypic effect 

 of a genetic difference. In any randomly 

 chosen case, however, one cannot predict to 

 what degree any particular phenotype will 

 be influenced by the genotype and by the en- 

 vironment. So theoretically both phenotypic 

 similarities and phenotypic differences be- 

 tween two individuals of the same species 

 could result from each one of the following 

 four combinations: 



1. Identical genotypes in near-identical envi- 



ronments. 



2. Different genotypes in near-identical envi- 



ronments. 



3. Identical genotypes in different environ- 



ments. 



4. Different genotypes in different environ- 



ments. 



Following are specific examples of how 

 each combination can result in either pheno- 

 typic difference or phenotypic similarity: 



I. Identical genotypes in near-identical en- 

 vironments: 



Phenotypic difference — one small and one 

 medium sized bean from the same pure 

 line. 



Phenotypic similarity — two small sized 

 beans from the same pure line. 



