178 INBREEDING AND OUTBREEDING 



rarely occurs, and even when it does occur, it may 

 be merely an appearance rather than a reality. The gen- 

 eral consensus of opinion at the present time is that there 

 is no such thing as perfect dominance, that the hetero- 

 zygote merely approaches the condition of one or the other 

 parent more or less closely. When two different poten- 

 tialities are contributed by the parents, there results an 

 interaction between them and the end product is repre- 

 sented in the organism. Because the most striking effect 

 may resemble the character of one parent more than the 

 other, we say that this character is dominant. In reality, 

 in the more fundamental characters, the hybrid usually 

 shows a resemblance to both parents. The more common 

 illustrations of dominance, such as fur colors and flower 

 colors, probably have little to do with heterosis. Other 

 dominant characters, however, have a fundamental effect 

 upon development, nearly always being essential to great- 

 est vigor. Various grades of albinism are common in 

 maize and in many other plants. Since this affects the 

 amount of chlorophyll, the presence of albinism in any 

 form seriously retards the growth. In extreme cases the 

 plants are to'tally incapable of continuing existence be- 

 yond the stage made possible by food stored in the seed. 

 In animals, albinism does not have the physiological sig- 

 nificance that it has in plants, but even here it is some- 

 times unfavorable to the individuals showing it. In every 

 case, and in all degrees, true albinism is recessive to the 

 normal condition. In maize, the heterozygous green plants 

 cannot be distinguished from homozygous green plants. 

 Many other unfavorable characters in maize are also re- 

 cessive. Absence of brace roots, bifurcated ears, dwarf- 

 ism, susceptibility to smut all behave in this way. 



