HOWARD AND BOWARD. 35 



Table XVIII. 

 The F 2 generation of crosses between tipped parents. 



Cross. N , ' f f Beardless. Tipped, 



plants. rr 



Biffen's hybrid £ * Pusa 6 o (3) 179 179 



(0) 163 .. 163 



Total 342 342 



The complete explanation of these ratios in the F 2 and the iso- 

 lation of the two factors concerned in the 15 : 1 ratio can only be 

 arrived at after the F 3 generation of this latter cross has been grown 

 and the results examined in detail. The simplest explanation 

 appears to be that in the bearded parent two factors are present, one 

 capable of producing short awns or tips only, the other resulting 

 in fully bearded plants, while in the tipped parents the short factor 

 only is present. On the presence and absence hypothesis, the 

 results of the various crosses would be represented as follows : 

 Assuming the fully bearded parent contains two factors B and T 

 (B representing the long and T the short factor), the tipped plants 

 contain only one factor T, while the absolutely beardless plants 

 contain neither, then — 



(1) Fully bearded wheats would be represented by B B T T. 



(2) Plants with tips only would be represented by b b T T. 



(3) Absolutely beardless plants would be represented by 



bb tt. 

 The various crosses described above would then be represented 

 as follows : — 



I . Fully bearded (BBTT) x tipped (bbTT) would give in the 



F, BbTT and in the F 2 1 BBTT + 2BbTT + 1 bbTT. 

 2 Fully bearded (BBTT) x beardless (bbtt) would give 



BbTt in the Yi and the F, would be represented by 



the formula : — 



BBTT + 4 BbTt + 2 BbTT + 2 BBTt + bbTT + 2 bbTt + 



BBtt + 2Bbtt + bbtt. 

 3. Tipped (bbTT) x beardless (bbtt) would give in the F, 



bbTt and in the F, bbTT + 2 bbTt + bbtt. 



