1^8 Arend. L. HageJoorn and A. C. Hagedoorn. 



Cases in which a single quality is influenced by several genetic 

 developmental factors, are very common; they are the rule rather 

 than the exception. It would be difficult indeed, to point out a 

 character, on which only one gene had appreciable influence. We 

 must not forget, that only in those instances are we sure of the 

 exact influence one factor has on the development, in which we can 

 study this factor by comparing two individuals, to the development 

 of one of which only it has cooperated, whereas the other developed 

 without undergoing its influence. This holds true for genetic as well 

 as for non-genetic developmental factors. 



The only cases therefore, in which we can study the influence 

 of several genetic factors on the same quality, are those, in which 

 each of these genes is present in some, absent from other individuals. 

 Such cases are those of the grain-colour of wheat, of the coherence 

 of pea-seeds, eyecolour of the mouse, and others. In some of them, 

 as in the cases studied by Nilsson-Ehle, the factors studied all 

 happened to influence the development in the same direction. In 

 other cases some genes influence a quality in one direction, others in 

 an opposite direction. 



Lang showed that cases of so-called "blending" inheritance could 

 be easily accounted for, by the assumption that the character in 

 question were influenced by several different genetic factors, in 

 respect to which the two forms crossed differed. 



He showed, that by assuming the existance of si.x different factors, 

 each influencing the development in the same way, and to the same 

 extent, for which six factors the hybrid was heterozygote, the facts of 

 blending inheritance could be very well explained. There are however 

 one or two minor points which are somewhat strained in the way in 

 which he offered this explanation. 



In the first place, Lang assumed in his explanation, that the 

 quality in those individuals which are heterozygous for a given gene, 

 would be intermediate between the same quality in an individual 

 homozygous for this gene, and one lacking it. It is true, that there 

 seem to be undoubted cases in plants, in which individuals hetero- 

 zygous for a given gene, are distinguishabe from such, as are homo- 

 zygous for it, but such cases are very clearly exceptions rather than 

 the rule. And in animals, the Very few examples of the same pheno- 

 menon have a decidedly suspicious look; some of them have already 

 been shown to be cases, in which we had to do with two "anta- 

 gonistic" genes, and others look very much like cases of "mutual 



