286 FOUNDATIONS OF BIOLOGY 



From these few examples, selected almost at random from 

 the wealth of data at hand, it is clear that some cases of blend- 

 ing and mosaic inheritance, as well as alternative inheritance, 

 can be satisfactorily interpreted on fundamental Mendelian 

 principles. It is merely necessary to bear in mind that when 

 speaking of unit characters, we mean that the germinal 

 physical basis of characters, that is the genes which condition 

 their development, behave as units, for now we know that 

 some characters are determined by single genes, and some by 

 multiple genes. And further, that dominance is a relation 

 between a pair of genes rather than between their expressions, 

 characters. Therefore blending inheritance may be merely 

 an expression of the action of several pairs of genes, each 

 gene displaying dominance for one member of a pair; while 

 mosaic inheritance may represent the extreme where each 

 gene's influence is exhibited to the full in the hybrid. 



Within the past few years geneticists have been able by 

 the MULTIPLE FACTOR hypothesis to bring into line with the 

 Mendelian interpretation the inheritance of a large number 

 of characters, especially in the higher animals. Thus stature, 

 proportions of the parts of the body, build, as well as nearly 

 all of the physiological and mental characteristics in Man, 

 are evidently dependent upon multiple genes. This seems so 

 generally true in the higher animals and plants as to suggest 

 that their characters arc genetically relatively complex as 

 compared with those of many of the lower organisms. 



So it happens, as is usually the case, the more a 

 problem is studied the more complex it appears to become. 

 Suffice it to say that, although our idea of 'unit characters/ 

 'dominance/ and even 'segregation' is to-day somewhat 

 broader than Mendel conceived on the basis of his classic 

 experiments, it is evident that he supplied us with fundamen- 

 tal principles which are affording a common denominator for 



