INHERITANCE OF ABNORMAL VENATION. 21 



offspring than those which are abnormal in one wing only. There is no 

 relation between parents and offspring with respect to the side upon 

 which asymmetrical abnormalities occur. There is a correlation between 

 the two wings of individual flies with respect to the intensity of the 

 abnormality. 



(5) Normal male X abnormal female gives a greater percentage of 

 abnormal offspring than the reciprocal cross. 



(6) Not only has the abnormality increased in the abnormal strain, 

 but, in spite of artificial selection to the contrary, an increasingly large 

 number of abnormal individuals appeared for a while in the normal strain 

 and then with the same treatment the percentage again decreased. 

 When the flies are allowed to choose their own mates the percentage of 

 abnormals is kept low even when abnormal flies are added from time 

 to time. 



(7) Abnormality originally behaved somewhat like a Mendelian reces- 

 sive, but in the later generations departed, in its behavior, very far 

 from that theory as it is now understood. 



There would be little profit in reviewing the various modifications of 

 the simple Mendelian formula and pointing out in detail why they are 

 not satisfactory in the present case. I have tried most, if not all, of 

 those which have been proposed and also a number of original hypothe- 

 ses involving two or more allelomorphs. All these attempts have been 

 failures with the exception of the idea of variation of potency (Lutz, 

 1907). If sufficiently elaborated this will "explain" each of the con- 

 ditions set forth above, and until quite recently I believed that the 

 inheritance of the abnormal venation followed this modification of the 

 Mendelian law. It seemed quite probable that there was a single pair 

 of allelomorphs involved— the abnormality-producing factor and its ab- 

 sence — but that the strength of the positive one varied, and that these 

 variations were inherited, making the problem a combination of the 

 inheritance of a fluctuation variate and of Mendelian segregation (Lutz, 

 1908) . In my report at the time of finishing the work at Cold Spring 

 Harbor I even constructed hypothetical curves for this variation. How- 

 ever, I have since realized that the "explanation " of conditions 6 and 

 7 was very weak. It was "that in selecting parents to continue the 

 normal strain I merely selected flies having no extra veins. For the 

 most of the time the work of describing offspring was unavoidably so 

 far behind the breeding- work that I did not know what percentage of 

 their brothers and sisters were abnormal. Hence I had no way of judg- 

 ing as to the germinal constitution of the parents. The normal strain 

 is probably a mixture of flies lacking the abnormal factor (might be 

 called NN's) and of flies which have it in hybrid condition of weak 

 allelomorphic strength (NA's). In generations VII to xxn I was prob- 

 ably unconsciously breeding from these NA's. This is an answer to 

 the first part of condition 6. 



