CHAPTER VIII 

 ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 



The present chapter is designed to deal with those relationships in 

 which a single locus in the hereditary system is involved. 



Mendel worked with seven pairs of contrasted characters and he ob- 

 served that in all of these one member of the pair controlled the expres- 

 sion of the character when the individual was heterozygous. When tall 

 peas are crossed with dwarf the hybrid is tall, in fact slightly taller even 

 than the tall parent. Similarly yellow cotyledons are dominant over 

 green, and smooth over wrinkled seed. The same is true for the other 

 four pairs of characters. So important did this fact of dominance appear 

 to investigators that for some time after the rediscovery of Mendelism 

 reference was very generally made to the law of dominance, and great 

 significance was attached to any failure to observe dominance in genetic 

 investigations. But subsequent investigations have shown that domi- 

 nance, far from being a general rule, is merely a special condition met 

 with in certain cases of inheritance. That it is by no means universal 

 must be conceded. How far it obtains and what other conditions are 

 met with in its absence, we shall endeavor to show in what follows. 



Dominance is a relation existing between a factor and its allelo- 

 morph such that in plants heterozygous for the factor in question the 

 character expression is the same or approximately the same as that when 

 the factor is homozygous. Dominance, therefore, applies only to rela- 

 tions existing between a pair of factors. That two contrasted characters 

 show an intermediate condition is no evidence in itself that dominance is 

 lacking. It must further be demonstrated that this condition is due to 

 the fact that the character expression of a genotype Aa lies between that 

 of A A and oa. Otherwise the intermediate expression of the hybrid 

 character may be the expression merely of the action of several pairs of 

 factors each displaying dominance for one member of each pair, but 

 together giving an intermediate expression. 



The Extent of Dominance. Off hand it would appear that com- 

 plete dominance is a very common phenomenon in genetic investigations. 

 The seven pairs of contrasted characters in peas could hardly have dis- 

 played it in all the pairs unless it were a condition of wide occurrence 

 and considerable significance. Otherwise we should have to consider 

 this a remarkable case of coincidence. Likewise the oft-cited investi- 



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