TYPES OF FACTOR INTERACTIONS 



165 



tween rose and single and pea and single, but that the pair of factors 

 involved must be different in each case. This is again shown in crosses 

 between walnut and rose or pea-comb fowls for such crosses give walnut 

 in FI and 3 : 1 segregation in F 2 . Accordingly taking walnut as a domi- 

 nant type, rose comb may be conceived of as differing from it by the 

 factor r, and pea comb by the factor p. Walnut comb would then 

 contain the dominant allelomorphs RRPP, and rose comb would be 

 of the genetic constitution rrPP, and pea comb, RRpp. Since single 

 comb again differs by one factor from both rose and pea comb it must 

 be of the genetic constitution rrpp. This analysis explains the experi- 

 mental results which we have thus far outlined, but the critical test of 



RP 



Rp 



rP 



rp 



RP 



Rp 



rP 



rp 



FIG. 76. Checkerboard analysis of theoretical expectations in Ft from a cross between 

 rose-comb fowl (rrPP) and pea-comb fowl (RRpp). 



the hypothesis lies in the cross rose X pea. This should give walnut, 

 RrPp, in FI and in F z all four types in the proportions 9 walnut: 3 

 rose: 3 pea:l single as shown in the checkerboard in Fig. 76. In 

 one series of such experiments Bateson obtained the results shown in 

 Table XXX. 



TABLE XXX. INHERITANCE OP COMB TYPE IN FOWLS. 



In this table the results of the back cross of FI walnut to single are 

 also given. A comparison of the values given with those expected on 



