Genetic Material and Mitosis 



called the phenotype. Environmental differ- 

 ences can cause the same genotype to pro- 

 duce a variety of phenotypes, and one can 

 conclude that the differences between the 

 beans of a pure line are environmentally pro- 

 duced and are not due to differences in 

 genotype. 



Now consider another bean, of the same 

 species, which gives rise to offspring beans 

 (Figure 1-1 B) that are very large, large, 

 and medium sized. Since each of these pro- 

 duces offspring beans which again show the 

 same range of phenotypes. another and dif- 

 ferent pure line is clearly involved, within 

 which phenotypic variability is attributable 

 to environmental fluctuation. 



How can one explain the differences be- 

 tween these two different pure lines, one 

 producing some very small and small beans 

 and the other producing some very large and 

 large ones? Since all the beans are grown 

 under the same environmental conditions, 

 these phenotypic differences cannot be due to 

 environmental differences; instead they must 

 be due to genotypic differences. It must be 

 concluded, then, that the genetic material in 

 these two pure lines is different. How can 

 one explain that some of the seeds in both 

 of these genotypically different pure lines 

 are similar — medium sized? Apparently, 

 different genotypes have produced the same 

 phenotype due to the influence of the en- 

 vironment. 



As already mentioned, under similar en- 

 vironmental conditions the average size of 

 the beans produced within a pure line re- 

 mains the same regardless of the size of the 

 specific beans planted. That is, in the pure 

 line first described the offspring beans have 

 the same average size whether the very small 

 or the medium seed is used as parent. Sim- 

 ilarly, the average size of seed produced in 

 the second pure line is the same whether the 

 medium or the very large seed is the parent. 

 In other words, selection within pure lines 



is futile, as expected in view of the hypoth- 

 esis that all members of a pure line are ge- 

 netically identical. 



Throughout the bean experiments de- 

 scribed, every effort was made to keep the 

 environment the same. This does not mean 

 that the environment did not vary, but that 

 it varied approximately in the same ways 

 and to the same extent for all the groups in 

 the study. In this particular work it happens 

 that phenotypic variability due to the fluc- 

 tuations of environment is not so great as to 

 mask completely the phenotypic effect of a 

 genetic difference. In any randomly chosen 

 case, however, one cannot predict offhand to 

 what degree any particular phenotype will be 

 influenced by the genotype and by the en- 

 vironment. Hypothetically, then, two indi- 

 viduals of the same species can have both 

 phenotypic similarities and phenotypic dif- 

 ferences resulting from each of the following 

 four combinations, as the examples indicate: 



1. Identical genotypes 



in near-identical environments 

 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. 



2. Different genotypes 



IN NEAR-IDENTICAL ENVIRONMENTS 



Phenotypic difference — one small and one 



large bean from genetically different 



pure lines. 

 Phenotypic similarity — two medium sized 



beans from genetically different pure 



lines. 



3. Identical genotypes 



in different environments 

 Phenotypic difference — one bean plant 

 grown in the light is green, while an- 

 other grown in the dark is white, 

 though both are from the same pure 

 line. 



