Heredity - 509 



and this makes the genetic analysis more 

 difficult. 



In the case of eye color, which is relatively 

 insensitive to environmental influence, the 

 analysis has been quite successful. Pure blue 

 eyes are inherited as a simple recessive, but 

 the various darker shades of eye color are 

 determined by several other genes — the 

 darker alleles being usually dominant to the 

 lighter. Hair color and skin color are like- 

 wise determined by more than one gene pair. 

 In the case of skin color, all the ¥ 1 offspring 

 of a pure white-black cross display a fairly 

 uniform intermediate (mulatto) shade — 

 which shows that each of the original stock 

 is homozygous for the skin color determina- 

 tion. But the F L , children derived from first- 

 generation mulatto parents are of many 

 shades, ranging from pure black to pure 

 white. 



One analysis indicates that skin pigmenta- 

 tion in man is determined by two pairs of 

 genes, which, in the F x mulatto, are Aa and 

 Bb — the capitals indicating partially domi- 

 nant genes, which tend to make the pig- 

 mentation darker in an additive fashion. 

 Consequently the F 2 offspring of Fj mulatto 

 parents are distributed as follows: 



1 with 4 dominants — AABB (phenotypically 

 Negro) 



4 with 3 dominants — 2 AaBB and 2 AABb 

 (phenotypically dark) 



6 with 2 dominants — 4 AaBb, 1 AAbb, and 

 1 aaBB (phenotypically mulatto) 



4 with 1 dominant — 2 Aabb, 2 aaBb (pheno- 

 typically light) 



1 with no dominants — aabb (phenotypically 

 white). 



Such a situation allows for a graded type 

 of variation among the F 2 generation. In 

 fact, such cases where the same character is 

 influenced in the same way by two or more 

 independent pairs of genes acting additively 

 are generally designated as multiple-factor 

 characters, and the results have been referred 

 to as blended inheritance. Moreover, the 

 greater the number of independent hetero- 



zygous gene pairs involved in the determina- 

 tion of a single character, the less perceptible 

 will be the gradations among the offspring. 

 It is interesting to note, therefore, that many 

 characteristics in man — such as height, 

 weight, and intelligence — and many charac- 

 teristics in commercially valuable species — 

 such as milk production in cows and egg 

 production in chickens — are undoubtedly 

 inherited on a multiple-factor basis. 



An analysis of the inheritance of the fac- 

 tors determining the blood-group aggluti- 

 nogens (p. 325) in man and other mammals 

 indicates that these are transmitted as a set 

 of three allelic genes, namely, A, A B , and a. 

 Genes A and A n , which respectively deter- 

 mine agglutinogens A and B, are both domi- 

 nant to the recessive gene a, which gives rise 

 to no agglutinogens when it is homozygous. 

 However, neither A nor A B is dominant to 

 the other, so that the heterozygous combina- 

 tion AA D determines an individual belong- 

 ing to group AB. The inheritance of the Rh 

 agglutinogens, on the other hand, is deter- 

 mined by a more complex set of allelic genes. 



In addition to the cases cited previously 

 (p. 507), there are several other genes known 

 to be concerned with the hereditary trans- 

 mission of abnormalities in man. Polydactyly 

 (extra fingers and toes), brachydactyly (short- 

 ened fingers and toes), certain cases of dwarf- 

 ism, and a number of other relatively rare 

 and deleterious characters — all are trans- 

 mitted by single pairs of genes. Moreover, 

 the gene pairs concerned with the transmis- 

 sion of abnormalities tend to be recessive in 

 nature. To "remain in circulation," in fact, 

 such deleterious factors are quite apt to be 

 recessive, because dominant abnormalities 

 tend to be "bred out" of the species by a 

 natural tendency of most individuals to 

 choose a "normal" mate. 



The inheritance of mental and other less 

 tangible characteristics in the human species 

 constitutes an even more perplexing prob- 

 lem. In the case of mental qualities, particu- 

 larly, exact measurements are difficult to 

 make; and it is hard to disentangle genetic 



