HEREDITY AND VARIATION 



present if any pigment is to be deposited in the eye. Individuals who carry 

 two recessive genes in their cells are albinos; their eyes appear pink because 

 the blood vessels of the iris are not screened by pigment. Where pigment is 

 deposited, another pair of genes conditions its distribution. Homozygous or 

 heterozygous dominant individuals have a purple-black pigment behind the 

 iris and brown pigment in front of the iris and appear brown-eyed. Homo- 

 zygous recessive individuals have pigment only behind the iris and appear 

 blue-eyed. The albino genes affect pigmentation of the skin and hair, as 

 well as of the eye. Curly hair is dominant to straight hair. In color blind- 

 ness, which is the result of an inherited defect of the retina, and in hemo- 

 philia, a defect of the blood that prevents its clotting, the genes are 

 sex-linked (Fig. 6.22). The production of the A, B, M, N, and Rh antigens 

 found in human red blood cells is determined by heredity (pp. 65 and 164). 

 The Rh (D) gene is dominant, and individuals homozygous or heterozygous 

 for it contain the Rh antigen. In the A and B substances we have an ex- 

 ample of multiple allelomorphs. Three allelic genes are known to condition 

 the blood types, but only two of these genes occur in any individual. They 

 are symbolized as A (gives A antigen), a^ (gives B antigen), and a (gives 

 neither antigen). Individuals may, therefore, be genotypically AA or Aa and 

 have blood of type A; a^a^ or a^a and have blood of type B; Aa^ and have 

 blood of type AB; aa and have blood of type O. Among individuals of 

 European stock 45 and 42 per cent, respectively, type as O and A, only 3 

 per cent as AB. 



Mental as well as physical characteristics appear to be inherited. The 

 evidence seems to indicate that mental qualities leading to degeneracy, crime, 

 and pauperism, as well as those yielding leadership in all social fields, may 

 be inherited according to Mendelian principles. It is not to be understood 

 that there are special genes determining crime or pauperism but rather that 

 genes giving rise to defective mental equipment predispose to these undesir- 

 able social traits. In the same way the so-called inheritance of diseases is an 

 inheritance of morphological or physiological characteristics that render an 

 individual more susceptible to infections. Instances of prenatal infection 

 (p. 164) are not cases of inheritance as the term has been used in this chapter. 



The environment in which a particular gene complex develops may limit 

 and obscure its possibilities. Thus, a given combination of genes in a human 

 individual may produce a better adult in a favorable environment than it 

 could in an unfavorable one. On the other hand, a good combination of 

 genes will give rise to a better individual in a particular environment than 

 will a poor set of genes. Training is another element that is very important 

 in the unfolding of human potentialities. An inferior inheritance with su- 

 perior training may result in an individual better fitted for society than one 

 with a good inheritance and no training. But no amount of training can 

 produce anything for which the inherited capacities are not present, nor can 

 the best of environments implant qualities if the potentialities for them are 

 lacking in the germ plasm. In other words, heredity limits very definitely 



the effects of training and environment. 



^ 199 



