VARIABILITY IN LINKAGE OF CHARACTERS OF MAIZE 
27 
PROPORTION OF CROSSOVER TO NONCROSSOVER GAMETES NOT AFFECTED BY 
VITALITY OF POLLEN 
Early experiments indicated that crossing over in the male 
gametes was more variable than in the female. Since the male 
flowers are more exposed than the female, the male gametes are 
subjected to greater vicissitudes. The seasonal and climatic factors, 
therefore, might be expected to influence the rate of crossing over or 
to act as a selecting agency on the gametes after they were formed. 
Preliminary analysis of linkage data amassed for other purposes 
showed a rather high positive correlation between the number of 
seeds on an ear and the percentage of crossing over, when the pollina- 
tions were so made that the seed-bearing parent was homozygous 
recessive for the characters in question. This correlation seemed best 
explained as resulting from a death rate of the crossover classes 
4- cT 6 
fO 
Fig. 3. — Inheritance of percentage of crossing over, showing the ancestry 
of a strain having an extremely variable rate of crossing over, the 
variability increasing with the progress of generations 
in the male gametes. Subsequent material of a more homogeneous 
nature has failed to substantiate the magnitude of the correlation be- 
tween fertility and crossing over, but before these additional data 
were available it seemed desirable to test directly the effect of un- 
favorable conditions on the percentage of crossing over in the male 
gametes. 
The simplest plan and one in harmony with natural conditions 
seemed to be to apply fresh heterozygous pollen to the double-reces- 
sive parent, store the remainder, and apply it at different periods 
throughout the day. Accordingly two heterozygous plants were 
selected, and on several days samples of pollen collected from them 
in the morning were applied at various times. For a variety of 
reasons these preliminary investigations were not conclusive, but 
demonstrated a decreasing viability of pollen with the increase in 
time of storage, showing that the opportunity for selective action 
