DETERMINATION OF DOiMINANCE IN MENDELIAN 

 INHERITANCE. 



By CHARLES B. DAVENPORT, Ph.D. 

 (Read April 25, 1908.) 



The longer one investigates the phenomena of heredity the more 

 one is impressed with the grandeur of the discovery made over forty 

 years ago by Gregor Mendel. His method is not less important than 

 its results. Following him, in studying heredity one considers a 

 single character at a time. One notes the result in the ofifspring 

 when this character assumes contrasted forms in the two parents or 

 when one parent has the character and the other lacks it. Under 

 these circumstances one frequently, nay, usually, finds that the con- 

 dition in one parent dominates over that in the other parent, so that 

 the ofifspring are all alike, and like one parent, in respect to that 

 character. The opposite, or recessive, quality is not lost, however. 

 It persists in the germ plasm and one half of the germ cells of the 

 individuals belonging to the first generation of hybrids contain the 

 dominant and one half the recessive quality. 



Dominance, it will be observed, it a matter of the soma. The 

 hybrid fertilized egg contains both contrasting qualities and so, 

 probably, do all of the cells of the body. But only one of the quali- 

 ties ordinarily makes its appearance. It has been suggested that a 

 struggle occurs between the contrasted qualities and the stronger — 

 called the dominant — wins. The question is what determines this 

 assumed greater strength of the dominant quality ? What determines 

 dominance ? 



Various replies have been given to this question. It has been 

 suggested that the dominant quality is the older and although this 

 is sometimes true it so often fails to be so that age cannot be 

 regarded as the primary cause of dominance. Frizzling and silki- 

 ness of fowd's feathers are each novelties but one dominates over the 

 ordinary flat feather and the other is dominated by it. Much evi- 



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