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, T:, from 
both parents, even though the latter both carry it; 
Tz, in other words, acts as a lethal when homozygous 
and so a pure stock cannot be obtained. T;, 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 T; as 
well as for T,. 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 
