68 



MENDELISM 



CHAP. 



white bird by /, then we must write the constitution 

 of the recessive white as ccii, and the dominant white 

 as CCII. We may now work out the results we 

 ought to obtain when a cross is made between these 

 two pure white breeds. The constitution of the F 

 bird must be Ccli. Such birds being heterozygous 

 for the inhibitor factor, should be whites showing 

 some coloured " ticks." Being heterozygous for 

 both of the two factors C and /, they will produce 

 in equal numbers the four different sorts of gametes 

 CI, a, cl, ci. The result of bringing two such 

 similar series of gametes together is shown in Fig. 

 13. Out of the sixteen squares, twelve contain /; 

 these will be white birds either with or without a few 



coloured ticks. 

 Three contain C 

 but not /; these 

 must be coloured 

 birds. One con- 

 tains neither C nor 

 T ; this must be a 

 white. From such 

 a mating we ought, 

 therefore, to obtain 

 both white and 

 coloured birds in 

 the ratio 13:3. 

 The results thus 

 r V ^ ■ theoretically de- 



Diagrara to illustrate the nature of the F2 generation ^ 



from the cross between dominant white and duced Were foUnd 

 recessive white fowls. 



to accord with the 

 actual facts of experiment. The F birds were all 

 " ticked " whites, and in the F generation came white 



Fig. 13. 



