A Simple Chemical Device 



147 



which may be represented by GG, YGG, and YYGG, and the 

 only difference in these plants is in the number of Y genes thev 

 contain. Taking the negative homozygote, GG, we make the 

 heterozygote by adding one Y gene, and the positive homo- 

 zygote by adding two Y genes. If Y is dominant over its ab- 

 sence, and this is much the commonest form of Mendelian in- 

 heritance, the YGG individual looks like the YYGG, i. e. both 

 will have yellow seeds. If the absence of Y were dominant over 

 its presence the YGG individual would look like the GG individual, 

 both having green seeds, and if dominance were only partial, 

 the YGG individual or heterozygote would be intermediate be- 

 tween GG and YYGG, having greenish-yellow seeds. The 

 important point to emphasize is that the heterozygote and 



Fig. 1. Material needed for illustration of Mendelian inheritance. 



positive homozygote are the same as the negative homozygote, 

 with one or two portions of something (a Y gene in this case) 

 added. 



Material needed for the demonstration: Solution of litmus, 

 sodium hydroxide or aqua ammonia, hydrochloric acid; 3 half- 

 liter flasks, glass graduate, 1 dozen 15 cm. test-tubes, and two 

 test-tube stands. Also, if convenient, a blue and a red flower 

 of any species, and if these flowers are at hand, an alcohol lamp, 

 or a bunsen burner if gas is available, ring-stand, wire gauze, and 

 several small beakers. See Fig. 1. 



