202 READINGS IN BIOLOGICAL SCIENCE 



the conclusions taken for granted in the application of genetics to man 

 are the following: 



(i) The biological basis for the dozen or more major kinds of hereditary be- 

 havior has been adequately established by experimentation in animals and plants. 



(2) Man fulfills the biologic requirements for being subject to the same laws 

 of heredity as other organisms. (Among these requirements are sexual reproduc- 

 tion, a chromosome mechanism in which the chromosome number is reduced to 

 half in the sperms and eggs, physiologic processes similar to those of other organ- 

 isms, etc.) 



(3) Hereditary factors are associated with the chromosomes. The evidence 

 for this now amounts to what is practically a complete proof. 



(4) Mental traits have their basis in physico-chemical structure, and are sus- 

 ceptible to the same laws of heredity as other characters. 



(5) Heredity and environment are cooperative in the production of any 

 finished character. One or the other influence may in certain circumstances ap- 

 pear neghgible, but the dual nature may always be demonstrated. 



Let us turn for a few moments to the principles involved in the analysis 

 of human pedigrees. 



The transmission of diverse hereditary factors from one generation to 

 the next involves a series of phenomena resulting finally in the visible 

 expression of characters in observable ratios. Most of the events in this 

 series develop in direct consequence of the laws of probabihty, the prob- 

 abilities being exactly determinable, thus making genetics more readily 

 amenable to mathematical analysis at the present time than any other 

 biological science. The included phenomena are as follows: 



( 1 ) The segregation of factors into germ cells. Segregation involves the 

 separation of the two members of a pair of factors when germ cells are 

 formed so that one member of the pair goes to one of the resulting cells, 

 the other member to the other. Thus half the germ cells will normally 

 contain one factor of the pair, half the other. If the two members of the 

 pair of factors are different, so that the individual is said to be heterozygous 

 for that pair, the germ cells will be of two sorts, in equal numbers, in re- 

 gard to that pair of factors. Thus the probability that any given germ cell 

 of a heterozygous individual will contain a particular factor is one half. 

 However, abnormal segregation is known, in which certain factors do 

 not separate from each other, thus changing in these instances the prob- 

 ability of a given germ cell containing a particular factor. 



(2) The assortmejit of factors during segregation. If an individual is 

 heterozygous for two or more pairs of factors, the factors segregate at 

 random if they are located on different pairs of chromosomes. Thus, in 

 regard to two pairs of factors, four kinds of germ cells will be produced in 

 equal numbers; in regard to three pairs of factors, eight kinds in equal 

 numbers, and so on. The chance of a given cell containing any two par- 

 ticular factors is therefore one fourth, any three particular factors, one 

 eighth, and so on. However, if the factors are located on the same pair of 



