No. 586] THE INHERITANCE OF DOUBLENESS 



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hand, when the double-thrower is the pollen-parent, all 

 the F, progeny are heterozygous ; hence all the double- 

 thrower-pollen is double-carrying. These facts are il- 

 lustrated by the two following diagrams (adapted from 

 Gohlsclimidt) : 



Pi Double-thrower $ j Pure single d 



Fx Single (*) Single (*) 



F, Single Single (I) Single (i) Double (\) 



F 3 Single Single Single (i) Single (i) Doubled) 

 (pure) (pure) (pure) (heterozygous) (sterile) 



F, Single (i) Single ( .;) Double (J) 



In these two crosses, where the " singleness " in the F x 

 (or later) heterozygotes comes entirely from the pure 

 single parent, we get what seems to be an ordinary Men- 

 delian 3 result in F 2 ; the pollen of these heterozygotes 

 must carry both "singleness" and "doubleness." The 

 absence of singleness from the double-thrower pollen is 

 taken by Bateson (1914, p. 292, foot-note) as almost con- 

 clusive evidence of somatic segregation of factors, occur- 

 ring in such a way that the pollen-mother-cells receive 

 only doubleness. Neither he nor Miss Saunders, how- 

 ever, gives anv reason why singleness, rather than double- 

 ness, should be thus eliminated. Goldschmidt (1013) 

 and Belling (1915, p. 126) have stated that selective 

 sterility of pollen will also explain the case, and definite 

 evidence for this view is presented below. 



a In some crosses the proportion of doubles is smaller, possibly 1/16 



