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 F; and 3:1 segregation in Fx. Accordingly taking walnut as 2 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 ae rp 
RP RRPP RRPp RrPP RrPp 
Walnut Walnut Walnut Walnut 
Rp RRPp RRpp RrPp Rrpp 
Walnut Pea Walnut Pea 
rP RrPP RrPp rrPP rrPp 
Walnut Walnut Rose Rose 
rp RrPp Rrpp rrPp rrpp 
Walnut Pea Rose Single 
Fie. 76.—Checkerboard analysis of theoretical expectations in F: 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 F, and in F, all four types in the proportions 9 walnut:3 
rose: 3 pea:1 single as shown in the checkerboard in Fig. 76. In 
one series of such experiments Bateson obtained the results shown in 
Table XXX. 
TaBLE XX X.—INHERITANCE OF Coms TyPE IN FowLs. 
Cross | Walnut | Pea | Rose | Single 
Observed... 279 | 132 | 99 | 45 
RrPp X RrPp. we. 2 | Expected... 312 | 104 104 35 
Ration... .... 9 | 3 | o | dl 
‘Observed... 664 | 705 664 | 716 
RrPp Xrrpp.......-- | Expected... 687 687 | 687 | 687 
Ratio......! 1 | 1 1 | 1 
In this table the results of the back cross of 7; walnut to single are 
also given. A comparison of the values given with those expected on 
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