HEREDITY 323 



monohybrids Mendel demonstrated two rules governing the be- 

 havior of inheritable characters, (i) The law of Dominance and 

 Recession. Alternative characters are either dominant or recessive, 

 which means that in the case just given, all the peas in the Fi gen- 

 eration were Tall because Tall is dominant over Dwarf, Dwarfness 

 remaining latent in the presence of the character for Tallness. This 

 law has been found to apply to some characters but not to others. 

 (2) The law of segregation. In the F2 generation in the example 

 Tallness and Dwarfness separate out again after both have been 

 present together in the Fi generation. Moreover, neither character 

 has been changed because of its having been associated with the 

 other. This law has been found to be altogether sound in all cases. 

 Mendelian Laws and the Behavior of Chromatin. The 

 Gene. F'or an explanation of the mechanism and manner in which 

 these characters segregate we must now direct our attention to the 

 behavior of the chromosomes. The process is understandable if it 

 is supposed, to begin with, that the germ cell of one of the parents 

 carries a chromosome, P, with the character for Tallness on it. This 

 character determiner is called a gene. Suppose also that the germ 

 cell of the other parent carries a chromosome, M, on which is the 

 gene for the alternative character Dwarf. Genes which produce 

 alternative characters in the adult are termed allelomorphs, or 

 complementary genes. Now when fertilization occurs P and M 

 become members of a synaptic pair in the newly developing plant. 

 P has the gene for Tall and M the gene for Dwarf, but Tallness for 

 some unknown reason overshadows the gene for Dwarfness, that 

 is to say, Tallness is dominant. The resulting plant is TalL But in 

 its chromosomes it carries Dwarfness on the M member of a synap- 

 tic pair. Two terms are used to distinguish between what such a 

 plant or animal is in outward appearance, and what it contains as 

 its chromosomal constitution. The outward appearance is called the 

 phenotype; in the Fi generation here the phenotype is Tall. The 

 chromosomal constitution that describes the type of genes present 



