No. 511] "PRESENCE AND ABSENCE" HYPOTHESIS 419 



that the heterozygoses are yellow when young and rod 

 when they grow older. In this case the appearance of 

 yellow in the young stage leaves no doubt that this is the 

 fundamental color upon which the red is superposed. 

 The pure bred red snail— the positive homozygote is 

 red from its earliest stages. This latency of the red char- 

 acter in the young heterozygotes produces a dominance 

 of the absence of red over its presence in the early stages 

 of development, and if the snails are classified at this 

 time, the F 2 is found to consist of 3 yellows to 1 red. 

 Later in life the heterozygotes become red and the census 

 shows 3 reds to 1 yellow. 



The rather frequent occurrence of heterozygotes lack- 

 ing the usually dominant character may be quite appro- 

 priately said to present cases of latency due to the com- 

 bination of fluctuation and heterozygosis. 



Summary 



The " presence and absence" hypothesis assumes that 

 what appears to be a pair of characters in Mendelian in- 

 heritance is really the presence and absence of a single 

 character. This hypothesis has now won the support of 

 most of the leading experimental students of heredity. 

 The fact that the absence of certain characters dominates 

 over their presence has appeared to some to be a difficulty. 

 This paper shows that no such difficulty is involved and 

 simple chemical experiments are cited which, if dupli- 

 cated among plants and animals, as they no doubt are, 

 would give the dominance of absence over presence, with- 

 out recourse to * ' inhibiting factors. ' ' 



be said to show latency due to heterozygosis. This condi- 

 tion is exemplified by some of Lang's snail crosses. The 

 same phenomenon is involved in many cases of failure 

 of dominance in heterozygotes. 



Station fob Experimental Evolution, 

 Cold Spring Harbor, L. I. 

 December, 1908. 



