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PLANT GROWTH 



wrinkled yellow and round green. The original combination 

 of characters is known as the parental combination. The 

 new classes are known as the recombinations. The second 

 point to note is that the first Mendelian law still operates. 

 Notice that those plants with yellow seeds are 315 + 101 or 

 416 while the plants with green seeds number 108 + 32 or 

 140. This makes a ratio of 2.97 dominants to 1 recessive 

 or approximately 3:1. Similarly for the other characters, 

 there are 3 15 + 108 = 423 round and 101 + 32 = 133 wrinkled 

 or a ratio of 3.26 to 1. This again approximates a 3 : 1 ratio. 

 The ratio of the four classes together approaches 9:3:3:1. 

 This means that the two 3 : 1 ratios mentioned above operate 

 independently of each other. Mendel represented this con- 

 dition by a simple algebraic multiplication: 



(3A + a) (3A + a) = 9AA + 3aA + 3Aa + aa 



This may be simple algebra, but what does it mean? It 

 means simply this: by Mendel's first law we would expect 

 to get in F2 a ratio of three-fourths of the plants with round 

 seeds and one-fourth of the plants with wrinkled seeds. Now 

 when we take into account the other pair of characters we 

 find that they operate independently. Thus, we expect the 

 three-fourths of the plants which have round seeds to be 

 divided into two classes, 3 yellow to 1 green. Similarly, we 

 expect the one-fourth of the plants which have wrinkled 

 seeds to be divided into two classes, 3 yellow to 1 green. This 

 gives our four classes and the 9:3:3:1 ratio mentioned 

 above. Diagrammatically, the F2 population can be repre- 

 sented by a square: 



3/4 ROUND 1/4 WRINKUED 



3/4 YELLOW 



1/4 GREEN 



