148 



GENETICS AND EUGENICS 



and rose-combed varieties respectively, and when both P and 

 R are lacking the original type of single comb is formed. 



TABLE 18 



Unit-Characters of Domestic Fowls 



A. Sex-linked 



Dominant 



1. Black skin (silkies). 



2. Silver (lacing, spangling, penciling). 



3. Striped down of chicks, black breast 



of adult male (game bantams) . 



Recessive 

 Normal skin (dominant in femaL'S, 



imperfectly recessive in males),, 

 Gold (lacing, spangling, penciling). 

 Plain down, brown breast of male. 



Plants. No attempt will be made at a detailed survey of 

 unit-character variations in plants but certain general cate- 

 gories of variations may be indicated and examples cited. 

 These will serve to show that the same sorts of changes are at 

 work among plants as among animals to produce striking 

 varieties. 



1. Colors of flowers. Some of the clearest cases relate to the 

 colors of flowers. Wild species often exhibit in their flowers 

 a mixture of pigments associated in a definite pattern. Loss 

 or suppression of the pattern, or of one or more of its com- 

 ponent colors, leads to the formation of self-colored flowers. 



