CHAPTER VIII 



BLENDING INHERITANCE 



1. RELATIVE SIGNIFICANCE OF DOMINANCE AND 

 SEGREGATION 



OF the three fundamental principles which underlie 

 "Mendel's law," namely, segregation, independence of 

 unit characters, and dominance, the principle of domi- 

 nance has been found to hold true in a surprising num- 

 ber of cases and in relation to very diverse organisms, 

 notwithstanding the fact that its universal application 

 is by no means assured. 



Mendel himself noted certain exceptions to the law 

 of dominance, and his followers have pointed out with 

 increasing emphasis that it is subject to many modifi- 

 cations. It is now understood, indeed, that segrega- 

 tion, not dominance, is the most essential factor in the 

 Mendelian scheme. 



2. IMPERFECT DOMINANCE 



It frequently occurs that dominance is so imperfect 

 that a heterozygous, or simplex, dominant may be dis- 

 tinguished at once by simple inspection from a homo- 

 zygous, or duplex, dominant, whereas the test of cross- 

 ing with a recessive is necessary whenever dominance 

 is complete, as has been previously explained. The 



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