MENDEL'S SECOND LAW 



71 



are WwAa, and the four gametes are WA, Wa, wA, wa. 

 The table below gives the sixteen recombinations 

 of these gametes: 



Eggs 



Sperm 

 WA 



Wa 



wA 



WA 



Wa 



wA 



Any individual that has both recessives ww or aa is white. 

 There are 7 such classes to 9 that carry both A and W. 

 Lastly, a 15 :1 modification of the 9 :3 :3 :1 ratio is obtained 

 when an individual homozygous for both pairs of recessive 

 genes gives a different result from any other combina- 

 tion. Thus, Shull found when Bursa pastoris, with trian- 

 gular capsules, is crossed to one with round capsules, the 

 latter appears in F^ only once in 16 times. 



ASSOBTMENT OF ThBEE FaCTOES 



"When three independent factor-pairs are present the 

 numerical expectation can be directly derived from the 

 9:3:3:1 ratio in the same way that the latter was derived 

 from the 3 : 1 ratio. Thus : 



One pair of factors. 

 Two pairs of factors. 

 Three pairs of factors. 



