ix] Gametic Coupling 151 



occur seven times as often as the other two possible combi- 

 nations. We speak of this phenomenon as Gametic Coupling. 



The term "coupling" is strictly applicable, because the 

 association is between two dominant or " present " factors, 

 here those for blue colour and long pollen. Abnormal 

 distributions due to such coupling are to be carefully dis- 

 tinguished from those described later under the name 

 "Spurious Allelomorphism," to which the term "coupling" 

 should not be applied. 



I f the two pairs of factors are expressed thus : 



Dominant Recessive 



Blue colour B Red colour b 



Long pollen L Round pollen / 



the gametic series is not 



i BL + i 1+ \bL+\ bl, 

 but 7 BL + i 1+ i bL + j bl, 



or very nearly so. 



Such a gametic series would give an F^ family com- 

 posed thus : 



177 blue-long : 15 blue-round : 15 red-long : 49 red-round. 



Reference to Fig. 17 will show how these numbers are 

 arrived at. 



These ratios agree very nearly with those observed in 

 actual experiments. For example the following series has 

 been produced. 



Purple Red White 



Long Round Long Round Long Round 

 Observed 1528 106 117 381 1199 394 



Calculated 1448-5 1227 1227 401-5 1220-5 407-4 



The correspondence between calculation and observa- 

 tion is very close, and in the case of the one figure which 

 departs sensibly from expectation it may be suggested with 

 great probability that the observed excess of purple longs is 

 due to some amount of cross-fertilisation effected by the 

 leaf-cutter bee, Megachile. Such crossing would obviously 

 tend to increase this class at the expense of the rest. 



Study of the F 3 families has proved conclusively that 

 the abnormal distribution occurs only among the gametes 

 of plants which are heterozygous both in the pollen characters 



