232 



PRINCIPLES OF ANIMAL BIOLOGY 



and single comb differ in just one pair of genes, with pea dominant. In 

 like manner it is shown that rose comb (upper right) is dominant over 

 single and differs from single by just one pair of genes. 



What should be expected, then, if fowls showing the two dominant 

 characters pea and rose, respectively, are crossed? No one could pre- 

 dict the result; it has to be determined by experiment. The hybrid 

 proved to have a large rounded comb overhanging the base of the beak, 

 as in the center of Fig. 201. From its shape this comb is called walnut. 

 A clue to the nature of this remarkable character is obtained by breeding 

 some of the Fi walnut fowls together. They yield four kinds of offspring 



Fig. 201. — Interaction of genes for combs in fowls. The gene for pea comb (upper 

 left) interacts with that for rose comb (upper right) to produce walnut comb (center). Two 

 of these Fi walnut-combed fowls, bred together, produce four types of offspring. Single 

 comb (lower right) is produced when^ neither the pea gene nor the rose gene is present. 

 {Rearranged from Punnett, " Mendelism." Courtesy of The Macmillan Company.) 



walnut, pea, rose, and single (lower row, Fig. 201). Very significantly, 

 the ratio of these four types is 9 :3 :3 : 1 in the order named and pictured. 

 This ratio indicates that two pairs of genes are involved. The pea- 

 combed fowl must have had the formula PPrr, the rose-combed one 

 ppRR, in which P is the gene for pea comb, p for no pea, R iov rose 

 comb, r for no rose. Single, which is "no pea" and "no i-ose," is pprr. 

 The student is encouraged to work out the gene explanation for the 

 Fi and F2 generations; any individual possessing both dominant genes 

 P and R will have a walnut coml). Whatever effect these genes have, 

 singly, on the physiology of comb development, together they interact 

 to produce a very different effect. 



Many other examples of interaction of genes belonging to different 

 pairs have been discovered. Sometimes the relation is such that one of 

 the genes in question cannot produce a visible result unless the other 



