VARIABILITY IN LINKAGE OF CHARACTERS OF MAIZE 6 
dissolved or upon the certainty with which a given complex of linked 
characters can be relied upon to remain in combination. 
Geneticists working with Drosophila have demonstrated the exist- 
ence of man}' factors both heritable and environmental that affect 
the degree of relationship of certain genes. The data accumulated 
by the several investigators who have considered the problem of 
variability in crossover rate have been assembled and evaluated by 
Morgan, Bridges, and Sturtevant {13). 1 For a thorough discussion 
of the genetic and environmental factors that affect crossing over 
in Drosophila the reader is referred to these authors. 2 
The nature of the mechanism assumed, to provide a means for the 
transfer of heritable elements from one chromosome to another, is 
such as to render any very definite rates of transfer extremely im- 
probable yet genetic literature abounds in discussions that aim to 
fix within one or two units the distances that separate two genes. 
Those working with characters of mature plans where populations 
are limited to hundreds of individuals long have recognized the futil- 
ity of attempting to establish crossover rates other than to determine 
whether a given pair of genes is linked or independent. Where dis- 
tances are approaching 50 units even this modest effort is not always 
successful, and in many cases investigators have succeeded only in 
amassing conflicting data which for definite interpretation must 
await the discovery of characters located at the intermediate 
positions. 
With the seed characters of maize it is possible to go somewhat 
further, for with these characters large populations are matured 
under very uniform conditions, yet even here much variability is 
encountered. 
Investigators of linkage phenomena in maize are familiar with 
the fact that in tests involving a number of ears some will be found 
that depart from the crossing-over mean of the progeny by percent- 
ages too large to be ascribed to chance. This is especially noticeable 
in linkages involving seed characters where relatively large popula- 
tions are possible and the significance of the aberrant ratios can be 
established conclusively. 
In numerous progenies involving the linked pair of factors C and 
Wx* individual ears have been foimd having as low as 8 per cent 
and as high as 50 per cent of crossing over. In these cases the indi- 
vidual ears clearly were aberrant as far as the percentage of crossing 
over was concerned, and the observed percentage could not be con- 
sidered as due to errors of random sampling from a population hav- 
ing the percentage of crossover gametes indicated by the mean of the 
progeny in which they appeared. 
The material used for the most part has arisen from a single 
cross between the Chinese waxy maize and a colored pop corn from 
Algeria. Numerous progenies have been grown from the original 
cross in the course of experiments attempting to interpret aberrant 
aleurone, waxy, or crossover ratios, and though most of these prob- 
1 Italic numbers in parenthesis refer to " Literature cited," p. 64. 
2 Since this bulletin was prepared there has appeared a compi-ehensive study of cross- 
over variability in maize by Stadler (17). Although many of the conclusions here pre- 
sented are in accord with those of Stadler, in some instances there is disagreement. He 
also presents data on some features not considered in this paper. For a more complete 
understanding of the several points involved both papers should be consulted. 
3 G is a factor necessary to the production of aleurone color and Wx is the factor that 
differentiates horny and waxy endosperm. 
