NEW COMBINATIONS OF GENES 15 



GENETIC VARIATIONS RESULTING FROM NEW COMBINATIONS 



OF GENES 



There is in nature a great mechanism, always operating, for 

 the production of new combinations of genes, and thereby 

 giving rise to genetic variation on a grand scale. The opera- 

 tion of this mechanism is what we call biparental reproduc- 

 tion, with its consequences that we call Mendelian inheri- 

 tance. This is perhaps the most potent means that is known 

 to us for yielding genetic variation and promoting evolution- 

 ary advance. Its operation therefore we must sketch. 



Heredity is commonly thought of as a matter of resem- 

 blance, rather than of diversity ; resemblance between related 

 individuals. But Mendelian heredity produces diversity as 

 well as resemblance. Production of diversity is indeed perhaps 

 its most conspicuous feature; it might equally well be called 

 Mendelian variability. From a single pair of parents there 

 arise by Mendelian inheritance offspring of many diverse 

 types. Any two unrelated human parents doubtless have the 

 capability of producing offspring of a thousand diverse types. 

 Certain features of this process are of great interest for the 

 problem of genetic variation and evolution. 



Each individual, as we have seen, has a certain combina- 

 tion of genes, some dominant, some recessive ; half of them 

 have come from his mother, half from his father. This defi- 

 nite combination of genes results in producing, under the 

 given life conditions, a certain set of characteristics, structural 

 and physiological. They give a body of a certain size, form 

 and structure, with functions that are performed in a certain 

 way; with a certain type of mental characteristics and be- 

 havior. They give an individual with a definite personality. 



Now, when individuals reproduce, this combination of 

 genes is taken apart, and a new one is made by putting to- 



