2 GEORGE HARRISON SHULL. 



"positive homozygote." Every vegetative cell of the green- 

 seeded pea-plant contains the pair of units or "genes" * GG, 

 l.ut when the germ cells are being formed, these two genes 

 separate so that each egg-nucleus and each sperm-nucleus has 

 only one G, or green-producing gene. 



In the same manner, every vegetative cell of the positive 

 homozygote, or pure-bred yellow-seeded pea-plant contains 

 the two pairs of genes, Y YGG, since it received a Y and a G from 

 each of its parents, and all of its male and female germ-cells will 

 contain the two genes YG. Now, when a cross is made between 

 these two kinds of peas, we either apply a pollen-grain of the 

 green-seeded pea-plant to the stigma of the yellow-seeded pea- 

 plant and allow a germ containing a single G to find its way to 

 an egg-cell which contains YG, or we reverse the process, and 

 by applying the pollen of the yellow pea to the stigma of the green 

 pea, allow sperms containing YG to reach eggs containing only 

 G. The results of these two processes are the same, for GX YG= 

 FGxG, and every vegetative cell of the plants pioduced by such 

 a cress will contain the three genes, YGG, and as the green 

 seeded character, G, can not be seen when Y is contained in 

 the same nucleus, all of the hybrid plants of the first generation 

 from such a cross will have yellow seeds, and we say that yellow 

 is dominant over green. ** 



When these hybrids produce germ-cells the three genes, 

 YGG, separate again so that half the egg-nuclei contain G and 

 half contain YG and similarly half the sperm-cells contain G, 

 and half contain YG. When sperms that are mixed in this 

 way are applied to egg-cells having the same mixture, some G 

 sperms will fuse with G eggs, to form GG plants (negative homo- 

 zygotes), some YG sperms will fuse with YG eggs to form YYGG 

 plants (positive homozygotes), some G sperms will fuse with 

 YG eggs to form YGG plants (heterozygotes), and some YG 

 sperms will fuse with G ej-js to produce YGG plants (also hetero- 

 zygotes). Thus, it is seen that three kinds of plants are produced 



*Thi^ word has b?en proposed by Dr. W. Johannsen for the internal units or deter- 

 miTarts. upon who e presence the production of any simple or unit character depends. 

 It has the advantage of being short and of nuking no apparent assumption a^ to the 

 u lira ate nature or behavior of such determining factors. } 



**Professor Bateson suggested that we use "epistatic" and ' 'hypostatic" to represent 

 the tiiitioasYo bstweei th ysllow ind green characters, and in other similar cases. 

 Thu-s white yellow is dominant over its absence it is "epijtatic to green," and green is 

 'hypostatic to yeiiow." 



