246 MULTIPLE FACTORS 



fore made in which the distribution of the various 

 component factors of truncate received from the 

 two parents could be separately followed among 

 their offspring, by reason of the association of the 

 homologous truncate factors in the different parents 

 with different linked factors for other characters. 

 It was found by this means that none of the off- 

 spring may ever receive the chief factor, T2, from 

 both parents, even though the latter both carry it; 

 T2, in other w^ords, acts as a lethal when homozygous 

 and so a pure stock cannot be obtained. T3, it was 

 found, may exist homozygously, but in that case 

 causes a marked reduction of fertility. This would 

 tend to keep the selected stock impure for T3 as 

 well as for To. Such a stock should throw a much 

 larger percent of normals than was actually ob- 

 tained in the selected race; the relative deficiency 

 of normals in the latter was evidently due to the 

 presence of another lethal factor in the chromosome 

 containing the normal allelomorph of truncate (see 

 case of beaded below). 



The case of truncate is of interest not only be- 

 cause the results indicate that other non-conform- 

 able instances might be similarly explained, but also 

 because the new methods — involving linked "iden- 

 tifying factors" — which have been developed in 

 attacking it are singularly adapted to the solution 

 of such problems. The use of these methods has 

 been made possible by the information at hand as 

 to the arrangement of the factors in groups and as 

 to the frequencies of crossing over. Without such 



