174 



HEREDITY AND EUGENICS 



showing the same desirable recessive quahty. All 

 this implies an array of information more elaborate 

 than anyone possesses concerning his own ancestors 

 and more distant relatives, unless he has made 

 exceptional efforts to collect and compile the necessar^^ 

 data from ancestral records, portraits, and other 

 sources. It w^ill also be understood that although 

 I have here spoken of dominant and recessive 

 characters as though the difference betw^een them 

 were absolute, yet it is well known that in many 

 instances this is not really the case. It is quite likely 

 that in man the interrelations with other germinal 

 qualities may bring about variable dominance; and 

 the heterozygous combination of a pair of characters 

 much more often gives an intermediate result, or at 

 least one in which the recessive exhibits some tangible 

 effect of its presence. Even in the case of such 

 abnormalities as brachydactyly, which are commonly 

 treated as dominants, there is some evidence that the 

 homozygous condition of the abnormality is much more 

 extreme, and indeed non-viable (see p. 90). If this is the 

 case, it shows incidentally that the original mutation 

 must have been itself heterozygous. It also follows 

 that the heterozygous condition is intermediate be- 

 tween the normal and the homozygous condition of 

 the abnormality, although the latter is non-viable 

 and cannot reach mature development. Abnormali- 

 ties in which the heteroz3^gous condition alone exists, 

 since the homozygous is non-viable, are not perhaps in 

 the strict sense dominants, although they are usually 

 referred to as such. Factors which can only bring 

 about development when in the heteroz^^gous con- 

 dition are one t3^pe of the class of factors now spoken 

 of as lethals (see p. 196). Such mutations, which 

 are commonly spoken of as dominants in the hetero- 

 zygous condition, are really defects which affect the 

 offspring when present on only one side of the house. 



