98 GENETICS 



fourth unit D is added by one parent, there would be 

 not only the original six double combinations, but in 

 addition to these, AD, BD, and CD, that is, as many 

 more as there are unit characters with which the 

 new one may combine. 



Obviously, when individuals are made up of very 

 many unit characters, as, for instance, a thousand, the 

 addition of one new unit character will increase the 

 possible double combinations a thousand fold. 



2. GALTON'S LAW OF REGRESSION 



Galton was one of the first 1 to attempt to express 

 mathematically the relationship between parents 

 and offspring by means of treating statistically a 

 single unit character. According to Galton, a mathe- 

 matical expression of the relationship between two 

 generations should serve as a corner-stone of heredity. 



What Galton did was to take human stature as a 

 unit character in comparing 204 English parents and 

 their 928 adult offspring, because human stature is 

 not complicated by environmental influences and is, 

 consequently, a purely hereditary matter. 



Since female height is normally less than male 

 height, the two were reduced, for purposes of 

 comparison, to a common male denominator by 

 multiplying each female height by 1.08, which is 

 the average amount that the male exceeds the 

 female in height. There are always two parents con- 

 cerned in the sexual production of every offspring, 



1 " Hereditary Genius," 1869. 



