38 Genes and Characters 



geneticists were interested in the inheritance of insanity but could 

 make little progress in determining what genes were involved in 

 its production. More recently, the psychologist has informed us 

 that what has usually been termed ''insanity" may be any one 

 of twenty or more different conditions. It is obviously impos- 

 sible to treat twenty characters some of which may be due to 

 dominant genes and some to recessives, some of which may have 

 high penetrance while others have low, as one character and 

 expect to get a satisfactory genetic analysis. 



A number of inherited conditions in man are fairly well 

 known, and it appears that every part of the body and many 

 physiological and psychological processes can be affected by 

 genes or combinations of genes. Sometimes dominant genes are 

 involved and sometimes recessives seem to be responsible. Often 

 the condition is so difficult to analyze or so few cases have been 

 found that on the basis of present knowledge it is impossible to 

 determine the exact nature and number of genes involved even 

 though the available evidence points distinctly to an inherited 

 condition. Some of the more striking or more important dis- 

 coveries will be cited here, but no attempt will be made to give 

 a complete list of inherited human traits. 



Genes have been found which affect the color of the skin. 

 Probably the most familiar is the recessive albino gene which 

 is similar to the albino gene in other animals and completely 

 prevents the formation of pigmentation in the skin, eyes, and 

 hair. Other differences in skin color such as the skin of the 

 black and yellow races in contrast to the white race are usually 

 of interest to most people. In both these situations, multiple 

 genes seem to be involved. Negroes seem to differ from whites 

 by two pairs of genes. In both there is a lack of dominance, 

 and all four genes interact cumulatively. Thus an individual 

 with the genotype ^4.4. BB would be very dark, whereas an aa 55 

 individual would be white. Mulattoes with A A Bb or Aa BB 

 combinations would be dark, those with Aa Bb, AA bb, or aa BB 

 would be intermediate, and those with the genes Aa bb or aa Bb 

 would be between an intermediate and a pure white. Other modi- 

 fying genes might also operate to influence these main types. 

 The gene differences between the white and yellow races also in- 

 volve several pairs of alleles. 



