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THE AMERICAN NATURALIST [Vol. XL VI 



had previously shown that four genotypes of Bursa bursa-pas- 

 toris are the four Mendelian types corresponding to two inde- 

 pendent factors (AABB, AAbb, aaBB, aabb). In his paper 

 above cited he deals with a cross between one of these types 

 (aabb) with a genotype of Bursa Heegeri corresponding to the 

 type AABB. The factors A and B in this cross behave in the 

 usual Mendelian fashion, departures from expected ratios being 

 explained by variation in dominance in one of the families. But 

 the factor or factors governing differences in the seed pod of 

 these two species present departures from expected ratios that 

 are not fully understood. There is evidence that at least two 

 genes are concerned in this difference. If only one gene were 

 concerned the ratio between the two types of seed capsule in F 2 

 should be 3 : 1 ; if there are two genes, the ratio should be 15 : 1, 

 three genes, 63 : 1. The ratios observed in F 3 were 4.67 : 1 (in- 

 stead of 3:1), 15.6:1, 24:1 and 63.5:1. The latter ratio, ob- 

 tained in a rather large family (129 individuals), suggests three 

 genes. The first and fourth of the above ratios are rather wide 

 departures from expected ratios, and their meaning is not yet 

 clear. The matter is still under investigation. There seems to 

 be little doubt that Dr. Shull has added another case to the in- 

 teresting class of Mendelian characters that may be represented 

 by more than one independent gene, such as those found by Nill- 

 son-Ehle, in oats and wheat and by East in corn. 



A very interesting paper by Gortner, 16 giving further results 

 of his studies on melanin formation, appeared in the December 

 (1911) number of this journal. He was able to show the color 

 pattern in the Colorado potato beetle is due to the fact that the 

 chromogen is secreted only in certain spots, while the oxidizing 

 enzyme, which is of the tyrosinase type, is present generally in 



W. J. Spillman 



