No. 546] 



MENDELIAN PROPORTIONS 



351 



breeding to get rid of recessive characters which are not 

 wanted. 



Summary 



In the foregoing discussion an attempt has been made 

 to exhibit clearly the facts of stable ratios involving 

 Mendelian dominants, heterozygotes and recessives. 

 While suggested by a study of lefthandedness, what has 

 been said will apply to any recessive character which is 

 not selected against in mating and which does not affect 

 the success of the organism in other ways. I have tried 

 to accord full value to the mathematical features of the 

 case, and have pointed out the various checks which tend 

 to hold the population in a given ratio. Yet I have been 

 unable to escape the conclusion that recessive mutants, 

 unless inherently weak in some respect, must tend to 

 increase in numbers at the expense of original dominant 

 types. These conclusions are reached from a considera- 

 tion of the following points: (1) The greater ease with 

 which characters may be lost than gained. (2) The great 

 number of combined dominants and heterozygotes which 

 through mutation may reach a simpler condition as com- 

 pared with the small number of recessives and hetero- 

 zygotes which may be imagined as affording opportunity 

 for mutation to dominance. 12 (3) The more likely sur- 

 vival of recessives in an environment of changing con- 

 ditions in which now the dominant and now the recessive 

 is hard pressed to maintain its existence. 



"Unless the ratio of the three types is 1 : 2 : 1, when the opportunities 



