500 WHITE— STUDIES OF INHERITANCE IN PISUM. 



]\Iendel's Law. 



The fundamental principle of Mendelism is very simple and rests 

 upon the assumption that animals and plants are made up of units 

 (called factors, genes, determiners, etc.), and that these units rpay 

 separate in the formation of the "germ-cells" (pollen and eggs) of 

 the hybrid offspring without having had any permanent influence 

 upon each other. The assumption that such units or factors exist 

 is based upon experimental data derived from crossing two plants 

 or animals from true breeding strains differing in two or more char- 

 acters and the growing of at least three subsequent hybrid genera- 

 tions under approximately the same environment as the original two 

 ancestors of the cross. For example, when two strains of peas, one 

 constant for purple flowers and green cotyledons and one constant 

 for white flowers and yellow cotyledons, are crossed, the first or F^ 

 generation is uniformly all purple-flowered with yellow cotyledons. 

 Self-fertilized seed from any of these F^ plants, if sown in sufficient 

 numbers, will produce approximately 9Pfl.YC : 3Pfl.GC : 3Wfl.YC : 

 iWfl.GC plants, showing that the determiner for green cotyledons 

 in addition to separating from its F^ associate — the determiner for 

 yellow cotyledons — also is inherited independently of its ancestral 

 associate — purple flower color. Mendel himself regarded purple 

 and white flowers in peas as a pair of characters, one of which com- 

 pletely dominated the other. Geneticists now largely hold to the 

 presence and absence hypothesis, by which the purple is regarded as 

 due to the presence of a factor or determiner for purple in the one 

 strain and the white-flower character as due to the absence of this 

 determiner or factor for purple color. Data from genetic experi- 

 ments, most geneticists believe, are more simply expressed by the 

 presence and absence concept. 



Since the promulgation of Johannsen's genotype hypothesis, 

 many geneticists believe these Mendelian factors to be unmodifiable 

 by selection and selection itself to be but a process of sorting out or 

 freeing hybrid or mixed populations from heterozygosis. 



Mendelian Studies of Peas. 

 Sixteen years have elapsed since the study of heredity assumed 

 the dignity of a separate science under the name of genetics. Dur- 



