40 MASS STUDIES IN BUILD. 



variable are the offspring of the M X M mating, 4.06 ± 0.11, and this 

 leads to the conclusion that a large proportion of the M parents are 

 not merely heterozygous, but constitute a "pure race" of medium 

 build. The offspring of the S X F mating have a fairly small varia- 

 bility 4.13 ± 0.16, as befits a first generation (Fi) hybrid. On the 

 other extreme, we have the M X VF mating with a standard deviation 

 of 9.11 ±0.41. This large standard deviation is due chiefly to the 

 inclusion of one family (Ber-A) which contains 2 progeny of builds 

 79 and 103, weighing 180 kg. (400 pounds) and 215 kg. (475 pounds) 

 respectively. Otherwise, the variability of this mating is not extreme. 

 It is 5.25 ± 0.24. The next largest variability is from the VF X VF 

 mating, 6.31 ± 0.55, a variability that is due to the absence of any 

 important mode. The progeny of the VS X F mating are highly 

 variable, 6.18 ± 0.59, but this standard deviation has the largest 

 probable error of any except VS X S, so that great stress must not be 

 laid upon its exact position. In general, the progeny of matings with 

 2 or 1 F or VF parents, belong to the more variable group and those 

 with S (or VS) parents to the less variable group. The meaning of 

 this is clear to the geneticist who has dealt with multiple factors. It 

 indicates that some or all of the factors that make for fleshy build 

 dominate to a greater or less degree over the factors for slenderness. 

 The test of the regression of progeny toward mediocrity and the test 

 of the variability of the progeny of the various matings thus lead to 

 the same result — the factors for fleshiness are imperfectly dominant 

 over those for slenderness, and the latter probably lack some or all of 

 those factors that make for fleshy build. 



HYPOTHESIS. 



The foregoing brief studies of the progeny of classes of matings 

 suggest the following hypothesis : 



Fleshy build results from the action of several positive (dominant) 

 factors that make for stoutness, while slenderness results from the 

 absence of one or more of such factors, or is due to recessive factors. 

 Fleshy parents may, and frequently do, carry gametes which lack the 

 "fleshy" or carry the "slender" factor, while in slender parents for the 

 most part the gametes carry only the slender factor, hence the gametes 

 of slender parents are more nearly homogeneous. This hypothesis 

 may be further developed as follows: 



Assuming that there are two independent factors A and B for build, 

 then these may be found in different zygotes in the following com- 

 binations : 



