no 



PROBLEMS OF GENETICS 



owing to defective constitution (e. g., the albinos of Antirrhinum 

 studied by Baur, and the homozygous yellow mice). I am 

 rather inclined to suppose that in these examples of hybrids 

 breeding true we shall find a state of things comparable with 

 that to which we formerly applied the terms "coupling" and 

 "repulsion." In these cases certain of the possible combinations 

 of factors occur in the gametic series with special frequency, 

 being in excess, while the gametes representing other combina- 

 tions are comparatively few. In a recent paper on these cases 

 Professor Punnett and I have shown that these curious results 

 vary according to the manner in which the factors are grouped 

 in the parents. If A and B are two factors which exhibit these 

 phenomena we find that the gametic series of the double heterozy- 

 gote differs according as the combination is made by crossing 

 ABXab, or by crossing AbXaB. In a normal Mendelian case 

 the Fi form, AaBb, produces gametes AB, Ab, aB, ab, in equal 

 numbers; but in these peculiar cases those gametes which contain 



Partial repulsion 



from zygote 



of form 



AbXaB 



Partial coupling 

 from zygote 

 of form 

 ABXab 



Partial repulsion 

 from zygote J 

 of form 

 AbXaB 



Partial coupling 



from zygote 



of form 



ABXab 



ing 



-3M 2 (2M I) 2W I 2W I M 2 (2W I) 



