68 



MENDELISM 



CHAP. 



white bird by /, then we must write the constitution 

 of the recessive white as ccii, and the dominant white 

 as ecu. 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 

 Cly Ci^ 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 

 /; 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 

 ^. .„ , , , theoretically de- 



Diagram to illustrate the nature of the F2 generation "^ 



from the cross between dominant white and duCCd WCre foUnd 

 recessive white fowls. 



to accord with the 

 actual facts of experiment. The F^ birds were all 

 " ticked " whites, and in the Y^ generation came white 



Fig. 13. 



