50 APPLICATION OF PKINCIPLES OF HEEEDITY TO BEEEDING. 



varieties crossed differ in n particulars. In many cases we can 

 neglect a great many of these differences, because they are immate- 

 rial and deal only with those character pairs which are important. 

 Table VI shows what occurs when the parents differ in two respects. 

 The first generation is heterozygote with respect to both character 

 pairs. In the second generation nine types occur in the proportion 

 shown in Table VI. 



In the third generation we get a different result with self-fertilized 

 plants from that obtained in cross-fertilized plants, because in self- 

 fertilized species a het- 



Lengfhs of vert/ca/ //nes cutoff by carves s/?ovy 

 proporf/'on of types /n eac/j ge/?e/xrf/o/7. 



Types. 



'WWCC 



WWCc 



^WwCC 

 WwCc 



Irnvcc 



[jVwcc 



[wwCC 



\vwCc 



WWCC 



2 3 4 5 



Generat/ons. 



ero zygote when it 

 makes seed breaks up 

 into one-fourth pure 

 dominants , one - half 

 heterozygotes, and 

 one-fourth pure reces- 

 sives ; while with open- 

 pollinated species the 

 proportion of hetero- 

 zygotes produced by 

 heterozygotes will be 

 larger than one-half, 

 because of the intro- 

 duction of foreign 

 pollen. 



Let us first consider 

 what occurs in self- 

 fertilized species if we 

 make no selection at 



Fig. 4.— Graphic illustration of ten generations of a hybrid in a self- 

 fertilized species without selection to type: W, winter character; w, 

 absence of TF(i. e., spring character); C, club character (short, thick q^}]^ amOng the progeny 

 heads); c, absence of C (i. e., long-head character). Types arranged 

 so as to bring together those that resemble each other. The space 

 between the top horizontal line and the uppermost curved line shows 

 the proportion of type TnrCC present from generation to generation, 

 and the space beginning opposite each type formula shows the pro- 

 portion of that type. The population ultimately consists almost 

 entirely of the four homozygote types present. 



of the hybrid. Figure 

 4 shows what occurs in 

 this case. Suppose the 

 plant with which we 

 are dealing is a hybrid 

 between a long-headed winter wheat and a club or short-headed spring 

 wheat. In the second generation we get the usual nine types, as seen 

 at the left of figure 4, where F= winter character, 'w; = spring character, 

 C = club heads, and c = long heads. The nine types are arranged 

 in figure 4 so as to bring together those which appear to the eye to 

 be alike. Thus the types WWCC, WWCc, WwCC, and WwCc are 

 all winter clubs, since W is dominant over w and C is dominant 

 over c. WWcc and Wwcc are long-headed winter wheats, wwCC 

 and wwCc are spring clubs, while wwcc is a long-headed spring type. 



