228 



The Nature of and Changes in Genes 



new material on which these other factors can act. An accumu- 

 lation of independent mutations has often resulted in an organism 

 so distinct from its ancestors that it has been classed as a new 

 species. 



Fig. 73. Pistils of five types of Oenothera. Right, Oe. Lamarckiana of 

 the genotype SpSp BrBr. Next left, brevistylis, homozygous recessive for the 

 gene for short style, SpSp brbr. Second from left, supplena, homozygous reces- 

 sive for the gene for double flowers, spsp BrBr. Center, "outside in," the double 

 recessive spsp brbr. Genes sp and br are complementary for the outside-in 

 type of flower, which type is not merely the summation of the effects pro- 

 duced by the two genes alone. In each of these illustrations the petals 

 and sepals and the set of stamens have been removed, leaving only the 

 gynoecium or its homologue. The big central bud in "outside in" is there- 

 fore the homologue of the functional stjde of supplena and brevistylis. 

 Left, mutant pollicata, a homozygous recessive with a solid floral tube or 

 hypanthium. (Courtesy of Dr. G. H. Shull.) 



QUESTIONS AND PROBLEMS 



1. Are there any chemical differences between euchromatin and het- 

 erochromatin ? Can you suggest any methods to show whether genes are 

 located exclusively in euchromatic or in heterochromatic regions? 



2. Prepare to discuss viruses, bringing out what they are, what they 

 do, and where they might be found. Discuss the possibility of obtain- 



