. The physical basis of heredity . color. Another interesting two-pair case involves varietiesof the combs of domesticated breeds of fowls. There is adominant type called Rose (Fig. 41, c), which, bred tosingle (wild type. Fig. 31, a), gives Rose in F^, and 3 Roseto 1 Single in Fg. Another dominant type called Pea(Fig. 31, h) likewise gives Pea in F^ and 3 Peas to 1 SingleComb in F^. But when Rose is bred to Pea there is notproduced the wild type, as one might have anticipated, buta comb called Walnut (Fig. 31, <Z), that differs fromboth parental types. The character is due to the com-bined
. The physical basis of heredity . color. Another interesting two-pair case involves varietiesof the combs of domesticated breeds of fowls. There is adominant type called Rose (Fig. 41, c), which, bred tosingle (wild type. Fig. 31, a), gives Rose in F^, and 3 Roseto 1 Single in Fg. Another dominant type called Pea(Fig. 31, h) likewise gives Pea in F^ and 3 Peas to 1 SingleComb in F^. But when Rose is bred to Pea there is notproduced the wild type, as one might have anticipated, buta comb called Walnut (Fig. 31, <Z), that differs fromboth parental types. The character is due to the com-bined action of both dominants. If two Fi birds withWalnut combs are bred to each other they give 9 Walnut,3 Pea, 3 Rose, 1 Single comb. This ratio shows that twofactors are involved, and that the Walnut comb appearsin all birds carrying both the Rose and the Pea Single comb is the double recessive form. MENDELS SECOND LAW 69 If the single comb be supposed to be the wild type,then Pea and Rose represent dominant mutant ^ Fig. 31.—Combs of fowls, single, a: rose, h; pea, r; and walnut, d. Neither produces any single comb, if the races are homo-zygous for Pea or for Rose respectively, but when crossed,the Pea comb brings in the normal recessive allelomorph 70 PHYSICAL BASIS OF HEREDITY of Rose, and the Rose comb the normal recessive allelo-morph of Pea: but the result is not an atavistic normalcomb, but a Walnut produced by the action of both domi-nants that are here the mutant characters. An important class of factors that are known asdiluters or intensifiers are often met with in geneticwork. For instance, a black mouse pure for a certaindiluting factor has a blue color (just as a blackmouse pure for albino factors is white). Such a bluemouse crossed to black gives F^ black mice, and in F2three blacks to one blue. A two-factor cross results whena blue mouse is bred to a chocolate (:^black cinnamon)mouse. The Fi will be black, the F. will be 9 black, 3 blue,3 choc
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