244 ESSENTIALS OF ZOOLOGY 



Complementary Genes. — In a number of cases color is produced 

 by two genes, the dominants of which must react on each other to 

 produce the color effect. In the sweet pea, if the dominant factor of 

 the red allelomorph (Rr) is acted on by the 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 pro- 

 duced, and if two white-flowered plants of the genetic constitution 

 Re and rC are crossed, a part of the progeny will be red. 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 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 for blue 

 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. 



White + White = Purple RcB + rCb = RGB 



Red + White = Purple RCb + RcB = RCB 



Purple + Red = Red RCB + RCb == RCl) 



Purple + White = Purple RCB + rcb = RCB 



Purple + White = White RCB + rcb = rcB 



Purple + White = Red RCB + rcb = RCb 



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 Fj generation, these cases 

 seem to produce perfect examples of blending inlieritance, but the F2 

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

 ceptional cases are often used to explain Mendel's law to beginning 

 students. 



The Andalusian breed of chickens includes both black and white 

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



