LINDSTROM, GENETICAL RESEARCH WITH MAIZE 349 
linkage between these factors, there being about 45 per cent crossing 
over. 
A 45 V 
VIT 

Other factors that have been fairly well tested against many others, 
such as 7a, gs, and 7, may form still other groups. Eyster (1922c) reports 
that zg and gs are completely linked and these may form another lin- 
kage group. 
It is an interesting observation that the factors representing charac- 
ters related in development, such as the various aleurone, plant color, 
the chlorophyll factors, exhibit no general tendency to be carried on 
the same chromosome. The aleurone factors A, C, R are located on se- 
parate chromosomes, as are the plant color factors A, B, Pl, andthe 
chlorophyll factors L, V, W. 
RELATIVE FREQUENCY OF CROSSING OVER IN MICROSPOROGENESIS 
AND IN MEGASPOROGENESIS 
Crossing over in maize between genetic factors in the same linkage 
group in both microspore and megaspore development was first recog- 
nized by COLLINS (1912) and proven by LINDSTROM (1917). The fre- 
quency of crossing over in the anthers and ovaries has been compared 
by Emerson and HUTCHINSON (1921) and Eyster, W.H. (1921 and 
1922a). 
Since linkage values of different plants of maize are known to be so- 
mewhat variable, the critical tests of the frequency of crossing over in 
the anthers and ovaries were made by using the same F, plant as the 
seed and pollen parent in backcrosses with double recessives. No con- 
clusive difference was found by these experimenters in the frequency 
of crossing over in megaspore and microspore development in the 
Tu-Su group, in the B-Lg group, or in the C-Sh group. 
EFFECTS OF INBREEDING IN MAIZE 
Maize is perhaps the best example of the action of the inbreeding 
process on a naturally cross-fertilized species. This is primarily true 
because of the large number of hereditary characters that have been 
isolated by inbreeding, whose inheritance has been studied thoroughly. 
It is commonly believed that maize, under ordinary circumstances, 
