THE STATISTICAL STUDY OF HEREDITY IN MAX 471 



galton's statistical studies of heredity in man 

 Galton was the first to make extensive use of the statistical method 

 of studying the degree of heredity existing between parents and off- 

 spring and between sibs. 



His first investigation concerned itself with the heredity of stature, 

 a highly variable character in man. He tried to find just how nearly 

 parents and offspring are correlated as to adult stature. Certain dif- 

 ficulties were met at the outset. First, there is a pronounced sexual 

 dimorphism in stature, males being considerably taller on the average 

 than females. To get rid of this difficulty, he first determined that 

 males are, on the average, about 1 inch to the foot taller than females, 

 and then transmuted all female statures into equivalent male statures 

 by adding an inch to the foot to the stature of each female. Second, 

 there are always two parents and they may be of very different stat- 

 ures, even after the mother's stature is transmuted into the male 

 equivalent. To overcome this difficulty, he averaged the statures of 

 each pair of parents and called it the mid-parent stature. The stat- 

 ures of the mid-parents were grouped into nine classes, ranging from 

 64 to 73 inches, each class having a range of an inch. Thus one class 

 consisted of those mid-parents falling between 72 and 73 inches, the 

 next class from 71 to 72 inches and so on down to the shortest class, 

 from 64 to 65 inches. The following schematic diagram (Fig. 88) 

 shows the extent to which offspring inherit stature differences from the 

 parents. If heredity were perfect, parent and offspring statures would 

 coincide on the diagonal line, but they do not coincide. Average 

 parents have nearly average offspring, but very tall and very short 

 parents have offspring decidedly less tall or less short, respectively, 

 than their parents. In the chart the arrow-heads indicate the points 

 where the average statures of offspring of each of the mid-parent classes 

 fall. Note that mid-parents which average a little over 64 inches have 

 offspring averaging about 6$h inches, and that mid-parents which 

 have an average stature of 71! inches have offspring averaging 70 

 inches. It is clear that, on the average, the more exceptional the mid- 

 parents are, the more the offspring regress toward the mean stature of 

 the group, or toward mediocrity. This is true of all but the extreme 

 tall group, where there is only a half inch of regression in the offspring. 

 The reason for this is probably associated with the fact that very tall 

 persons are homozygous (double recessives), shortness of stature being 

 incompletely dominant over tallness. 



Galton calculated that, with regard to stature, the offspring of 



