HEREDITY 339 



histories, illustrates two important principles: (i) Many mental and 

 physical traits are inheritable and appear to obey Mendelian laws; 

 (2) the desirability of the acquisition of more complete data on 

 human inheritance in general, to serve as a guide in various socio- 

 logical, legal, and medical programs. 



Concerning the possibility of predetermining sex, the facts at 

 present known indicate that the chief factor in determining sex is 

 the presence or absence of an X chromosome. Since in Man the 

 spermatozoa are of two sorts in equal numbers, one-half containing 

 and the other half lacking the X chromosome, and since the union 

 of the tgg and sperm is a chance contact in which the probabilities 

 of one or the other type of sperms fertilizing the egg are exactly 

 equal, the possibility of predetermining sex is remote. It would 

 involve controlling the type of sperm that is to reach the ovum first. 



Characters Not Represented in the Zygote Are Not 

 Transmitted. One of the prominent questions raised by biolo- 

 gists of the nineteenth century, and earlier, that is still prominent 

 in many fields of human thought, is the problem of the inheritance 

 of acquired characters, characters that are acquired by the soma and 

 not in any way derived from parents. With the facts of the role 

 played by the chromosomes before us, a preliminary statement may 

 now be made; more complete discussion is reserved for a later 

 chapter. In order to be transmitted, a character must be represented 

 on the chromosomes by genes. Therefore the only way in which a 

 character acquired by the soma, for example proficiency in a trade, 

 education, bad habit, crippling by injury, and so on, can become a 

 part of inheritance is by effecting changes in the genes, either 

 causing chemical processes to occur which construct new genes, 

 modify genes already present, or destroy genes. With an exception 

 noted farther on, in no case has it been possible to demonstrate by 

 experimental methods that somatic characters or the influence of 

 the environment affect the quality and behavior of the genes. Con- 

 fusion has arisen from failure to recognize the distinction between 



