830 TEXTBOOK OF ZOOLOGY 



dominant factor of the color allelomorph (Cc), then red is produced. 

 If either of these dominant genes is present without the other, white 

 flowers are produced, and if two white-flowered plants of the genetic 

 constitution Rrcc and rrCc are crossed that part of the progeny will 

 be red in which both R and C are inherited. A student familiar 

 with elementary chemistry may understand this better if he thinks 

 of the red factor as being represented by a colorless solution of 

 phenolphthalein and the color factor as a colorless alkaline solution. 

 When both solutions come together, red is produced, but neither solu- 

 tion can produce the color without the other. 



Supplementary Genes. — But sometimes purple sweet peas appear 

 when two white varieties are crossed or when a red variety is crossed 

 with a white variety. Purple is produced by a third gene which 

 intensifies red if it is already present, but has no effect unless the 

 RC genes are likewise inherited. Thus we find that its effect is to 

 supplement that of the two genes R and C which are complements to 

 each other in the production of red. The accompanying list shows 

 some of the surprising crosses that may be made with sweet peas. Of 

 course the combination of genes shown in the last column is not the 

 only one that will come about in each of the various cases, but it is 

 the one that emphasizes the point desired. Bb represents the in- 

 tensifying allelomorph. 



White + White = Purple RcB + rCb = RGB 



Red + White = Purple RCb + RcB = RCB 



Purple + Red = Red RCB + RCb = RCb 



Purple + White = Purple RCB + rcb = RCB 



Purple + White = WJiite RCB + rcb = rcB 



Purple + White = Red RCB + rcb = RCb I 



Lack of Dominance.— In several classic cases, some of which are 

 among plants and others among animals, neither factor of an allelo- 

 morph is dominant over the other. In the F^ generation, these cases 

 seem to produce perfect examples of blending inheritance, but the Fa 

 generation exemplifies the 1 :2 :1 ratio so beautifully that these excep- 

 tional cases are often used to explain Mendel's law to beginning stu- 

 dents. 



The Andalusian breed of chickens includes both black and white 

 individuals. When black fowl are crossed with white fowl, all the off- 

 spring are of a slate color technically known as ' ' blue. ' ' When a blue 



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