164 



GENETICS IN RELATION TO AGRICULTURE 



be interested in tracing out others. Thus the presence of a factor for 

 dominant white may be demonstrated by crossing with a purple race, 

 in which case the grains will be white, if such a factor be present. More- 

 over, several of the whites when crossed give colored forms in Fi, thus 

 11 X 13, 11 X 14, and 12 X 13 give purple, and 12 X 14 gives red. 

 It has already been shown how in case of the presence of the factor for 

 dominant white, whites when crossed may give a white Fi and white, 

 purple, and red in various proportions in F?. Such is the case for ex- 



Fio. 75. Comb types in poultry. Single, a; pea, b; rose, c; walnut, d; and breda, e. 



(After Morgan.) 



ample in the cross 1X16 which will give FI white and F 2 in the ratio 220 

 white: 27 purple: 9 red. The complex relations here existing between 

 only three phenotypes is a very good example of the sort of problems 

 which must be solved by experimental genetics. 



Comb -characters in Fowls. A variety of comb-characters are found 

 in the domestic breeds of poultry and Bateson has made these the sub- 

 ject of an extensive Mendelian investigation involving the rearing of 

 over 12,000 individuals. The comb types involved are shown in Fig. 

 75. In this series of characters both rose and pea comb were found 

 to be dominant to single comb, and give in F z simple 3 : 1 ratios. These 

 relations obviously indicate that there is a single factor difference be- 



