No. 632] SHORTER ARTICLES AND DISCUSSION 



26!> 



viduals are even more within the possibility of chance fluctuation 

 than those of the non-yellows. 



Dunn states that linkage between Y and B "affords a satis- 

 factory explanation of the observed facts in harmony with other 

 cases of linkage. ' ' However, one of the essential points of link- 

 age is that members of a multiple allelomorph series are linked 

 to a given gene in the same degree. Cuenot, 2 Morgan, 3 Sturte- 

 vant, 4 and the writer, 5 have shown that the genes for yellow and 

 for agouti in mice are allelomorphic. Many investigators, in- 

 cluding Durham, 6 Detlefsen, 7 and the writer, 8 ' 9 have shown that 

 agouti and black are not linked in inheritance — yet according to 

 our present knowledge of linkage all genes in the same locus are 

 equally linked with any other given gene, and these crosses 

 should show linkage between agouti and black to a degree equal 

 to that of linkage between black and yellow. Dunn has not em- 

 phasized this point sufficiently. 



Furthermore, if there is any significance other than random 

 sampling in the peculiar ratios reported, there is another pos- 

 sible explanation, not considered by Dunn, which avoids hypoth- 

 ecating linkage between yellow and black. If a lethal factor was 

 closely linked with black in the particular family under consid- 

 eration, and if this lethal was effective in a heterozygous condi- 

 tion in non-yellow mice — but not in yellow mice, the observed 

 results would be explicable as follows : 



, Y equal yellow, 

 B equal black, 

 L equal lethal, 



y equal non-yello 

 b equal brown. 

 1 equal normal. 



Yy BL bl equals yellow heterozygous for black and lethal. 

 Forms gametes : 



■Archlr /vol. Exp. et Gen. (4), 



