THE 



AM E R I CAN N AT U RAL1 ST 



Vol. XLIV March, 1910 No. 519 



THE IMPERFECTION OF DOMINANCE AND 

 SOME OF ITS CONSEQUENCES 1 



It has long been recognized that in Mendelian heredity 

 dominance is frequently imperfect. Mendel himself ap- 

 preciated this fact, for lie demies dominant characters as 

 those "which are transmitted entire, or almost unchanged, 

 in the hybridization " and points out that the hybrids 

 between purple-red flowered and white flowered peas have 

 lighter flowers than the darker parent. Indeed, in his 

 letters to Niigeli he speaks of the hyhrirf-forni of a char- 

 acter, recognizing that the hybrid character is not always 

 that of one parent. Practically all hybridizers of the last 

 nine years, and especially Correns and Bateson, have 

 observed and laid emphasis on this imperfection of domi- 

 nance. I can only add my testimony to theirs. It will 

 be well to illustrate the general principle by some con- 

 crete examples. Bateson, and later Baur, found in 

 hybrids between the prickly and non-prickly capsuled 

 Datura that the prickles were much reduced in size. 

 Correns found the hybrids between green- and yellow- 

 leaved Mirabilis to have light green leaves; and in the 

 same plant, variegation, though recessive, may be detected 

 in the heterozygotes. Further, Mirabilis typica, 90 cm. 



