Volume 12 Xumber ! 



The Plant World 



A Magazine of General Botany 

 JULY, 1909 



A SIMPLE CHEMICAL DEVICE TO ILLUSTRATE 

 MEN DELI AX INHERITANCE. 



By George Harrison Shull. 



At the recent meeting of the Botanical Society of America 

 at Baltimore, Dec. 31, 1908, I performed an experiment to illus- 

 trate how the absence of a character may be dominant over its 

 presence. The experiment was of such simplicitv and appeared 

 to have such pedagogic value that I was asked to describe it 

 in the Plant World for the benefit of teachers. 



In order to make the bearing- of the experiment perfectly 

 clear, I will first give a brief statement of just what is involved 

 in Mendelian inheritance, when considered from the standpoint 

 of the "presence and absence" hypothesis, now generally ac- 

 cepted by the foremost experimental students of heredity. 



The presence and absence hypothesis assumes that in every 

 case in which two plants or animals which may be crossed to- 

 gether, differ from each other in a simple unit-character, their 

 difference from each other is to be described in the terms of 

 presence and absence of a single character and not as a pair of 

 contrasted characters. For example, when a yellow-seeded 

 pea is mated with a green-seeded one, instead of saying that the 

 contrasted characters are yellow vs. green ,we should say " yellow 

 vs. absence of yellow." The pure-bred yellow pea is believed 

 to possess all the elements contained by the green pea, with a 

 single pair of units added which makes it yellow instead of green. 

 If the green pea, which I call in such a case the "negative homo- 

 zygote," is represented by the letters GG, intended to indicate 

 that it has inherited its greenness from both its parents, then 

 the pure-bred yellow pea should be represented by YYGG, to 

 show that both greenness and the added unit for yellowness 

 was inherited from both parents. This forms what I call the 



