306 GENETICS 



6. HEREDITARY DEFECTS 



Undesirable hereditary traits are frequently defects 

 due to the absence of some character. For instance, 

 albinism, which occurs in several kinds of animals and 

 also in man in one out of every 20,000 individuals 

 (according to Elderton), is due to the absence of pig- 

 ment in the skin, hair and eyes. Albinic individuals 

 have poor eyesight because they are unable to stand 

 strong light, being without protective pigment in the 

 eyes. This peculiarity of albinism behaves as a reces- 

 sive character both in man and in other animals. An 

 albinic individual may, therefore, marry a normal indi- 

 vidual without fear of producing albino children, al- 

 though the children of such a mating would carry 

 heterozygous germplasm with respect to albinism, and 

 in cousin marriages might subsequently produce some 

 albino children. 



Davenport, in his work on "Heredity in Relation to 

 Eugenics," brings together a long catalogue of human 

 hereditary defects, although in most instances they 

 are extremely difficult of accurate analysis. This is 

 true, first, because these defects so often probably de- 

 pend upon a combination of determiners rather than 

 upon a single one, and, second, because the available 

 data are usually scattered and incomplete. 



Deafness, for example, is a defect which is heredi- 

 tary though exactly to what degree, it is at present 

 impossible to state. The following table taken from 

 the extensive work of Fay (1898) upon "Marriage of 

 the Deaf in America" gives some idea of the results of 

 different matings lumped together statistically. 



