CONTINUITY OF THE RACE 



599 



Can such information be of assistance in 

 predicting what one might expect in off- 

 spring of matings where the skin pigmenta- 

 tion varied in the two parents? Obviously 

 matings between the double recessive 

 white and a person with any degree of pig- 

 mentation will produce offspring lighter 

 than the dark parent. Matings between in- 

 dividuals carrying only one gene for pig- 

 mentation and appearing as light as many 

 non-Negroes may produce children darker 

 than either parent. On the other hand, they 

 have an ec^ual (1:4) chance of producing 

 children with no genetic trace of pigmenta- 

 tion. Such white children are double reces- 

 sives ( aahh ) and therefore can never trans- 

 mit pigmentation to their offspring. 



Normal frequency and multiple factors 



The character of skin color involving two 

 gene pairs in man is a relatively simple case 

 of multiple factors. As a rule such charac- 

 ters involve many more gene pairs, as, for 

 example, height in man, where ten or 

 more are influential. With three gene pairs 

 influencing one trait, the F2 comes out in 

 the ratio 1:6:15:20:15:6:1. With each 

 added gene pair the shape of the distribu- 

 tion approximates more and more closely 

 a normal bell-shaped frequency curve ( Fig. 

 24-11). When ten gene pairs influence a 

 single trait, the distribution coincides with 

 the normal distribution curve. In other 

 words, ten gene pairs are sufficient to pro- 

 duce a normal population in respect to one 

 trait whether it be height, weight, length of 

 neck, or degree of intelligence. If one meas- 

 ures the height of 10,000 adult white males 

 in America, he would find a range from 

 about 55 to 85 inches with an average fall- 

 ing around 68. There would be very few as 

 short as 55 inches and very few as tall as 

 85 inches but a great many around the 

 average of 68 inches. By plotting the num- 

 ber of people at each height against inches, 

 a normal distribution curve results (Fig. 

 24-11). This is exactly what is obtained 

 when the distribution is computed for 



height on the basis of ten gene pairs being 

 involved. 



Quantitative studies of populations 



Long before Mendel's Laws were known, 

 some effort was made to understand in- 

 heritance by a careful analysis of a single 

 trait in large populations. The most out- 



Fig. 24-11. Height in a population of humans shows a 

 normal bell-shaped frequency curve as shown in the 

 upper figure. There are a few about 55 inches tall and 

 a few about 85 inches tall; most people are between 

 these extremes, the average being about 68 inches. 

 When these heights are plotted against numbers, a 

 bell-shaped curve results, which is what is expected 

 when ten gene pairs are involved. 



The lower portion of the figure shows how Galton's 

 law of filial regression operates. Children of short and 

 tall parents are never as short or as tall as their 

 parents. They tend toward the average height of the 

 population. 



standing investigator in this field was Sir 

 Francis Galton of England, referred to ear- 

 lier, who studied the inheritance of many 

 traits of British people, including such in- 

 tangible ones as intelligence. He discovered 

 that high intelligence seems to "run in fami- 

 lies," as shown by the frequency that names 

 of members of famous families appeared in 

 the British "Who's Who." Such traits as 



