ADZUKI BEANS AND JIMSON WEEDS 



Favorable Class Material for Illustrating the Ratios of Mendel's Law-^Actual 

 Practice in Making Counts Is Necessary Before the Student 

 Can Fully Grasp Modern Ideas of Heredity 



A. F. BLAKIiSLKI-: AXD B. T. AVKRV 



Station for Experimental Evolution {Carnegie Institution of W'asliin^ton), Cold 

 Sprino Harbor, I^cng Island, X. V. 



GENETICS is a difficult subject 

 to teach without an adequate 

 supply of demonstration and lab- 

 oratory material. 



Mendel's law, especially, is in need of 

 illustration. It is like the law of valence 

 in chemistry in that considerable prac- 

 tice in working out ratios is necessar}' 

 before the student can appreciate and 

 use these laws of definite proportions 

 in predicting results. Counts of actual 

 Mendelian ratios in the field or labora- 

 tory are as essential as the manipulation 

 of test tube reagents in chemistry. 



Two plants — the Adzuki Bean and the 

 Jimson Weed — have been grown in the 

 genetic section of the botanic garden 

 at the Connecticut Agricultural Col- 

 lege and have shown themselves es]3e- 

 cially favorable material for illustrative 

 purposes in classes in genetics. 



The two forms have been used also 1j}' 

 several teachers to whom seeds ha\-e 

 been sent. Their interest in such mate- 

 rial seems to warrant the publication 

 of the present brief paper on the 

 availability of these plants for class 

 purposes. 



THE ADZUKI BEAN 



The Adzuki Bean (Phascohis Mungo) 

 is especially desirable as a form with 

 which to introduce the student to 

 Mendel's law. 



Pedagogically it is better, we believe, 

 to start with the 1:2:1 ratio rather than 

 with the 3:1 ratio. In the Adzuki 

 Bean, mottling in the seed coat is 

 dominant to lack of mottling. In the 

 hybrid or heterozygous condition, how- 



ever, the mottling is lighter than in the 

 pure or homozygous condition. Races 

 occur with red seed coats but the differ- 

 ence in mottling shows best on gra\' 

 seeds. Heterozygous plants, therefore, 

 can be readily distinguished from homo- 

 zygous plants by an inspection of the 

 seeds which they produce. It should 

 be remembered, however, that the seed 

 coats are maternal characters and do 

 not indicate the genetic composition 

 of the embryos which they enclose. 



Fig. 10 shows the expected results 

 from a cross between a plant bearing 

 dark mottled gray seeds and one bear- 

 ing unmottled gray seeds. The dark 

 mottled and the unmottled gray seeds 

 breed true when selfed while the light 

 mottles break up into a 1:2:1 ratio in 

 each generation. If the intensity of 

 mottling is disregarded this 1:2:1 ratio 

 Ijecomes the more familiar 3:1 ratio. 

 Fig. 11 shows the result that might 

 be expected from crossing a homozygous 

 dominant (dark mottled) with a hetero- 

 z}'gous dominant (light mottled) plant. 

 The offspring show all dominant if the 

 presence of mottling is alone considered 

 or a 1:1 ratio if the grades of mottling 

 are distinguished. Similarly in Fig. 

 12 are shown the expected results from 

 crossing a heterozygous (Hght mottled) 

 l)lant with a recessive (gray unmottled). 



For class purposes we have ne\-er 

 taken the trouble to make crosses since 

 experience shows that off -pollination 

 rarely occurs in these forms. ^ We have 

 grown them thus far three seasons and 

 have found that light mottled seeds 

 always segregate, giving a 1:2:1 ratio 



1 In one instance we found occa.sional plants apjjearing witli red seeds in a pedigree where 

 only grays were expected. Selfingthe plants in this pedigree showed that the white seed coat is 

 dominant to red seed coat and that off-pollination must have occurred in a previous generation. 



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