VAKIABILITY IN LINKAGE OF CHARACTERS OF MAIZE 
25 
Many more progenies having crossover percentages at all points 
between these two centers have been obtained, but nothing can be 
stated as to the stability of these intermediate positions. 
Although the best evidence now available indicates that heritable 
differences are involved in the variable percentages of crossing over, 
this conclusion has not been reached without many discordant facts 
and clear evidence that factors other than heritable ones also affect 
the rate of crossing over. In an effort to establish high and low rates 
of crossing over by growing representatives of the extremes, it was 
found that inheritance of the parental condition is the exception 
rather than the rule and that most of our progenies show complete 
regression to the mean of the parental progeny. Occasionally also 
the offspring of extreme parents show crossover percentages at the 
opposite end of the scale, in consequence of which it has been con- 
cluded that environmental factors sometimes obscure the true genetic 
condition. 
250b 
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698 
/4/j /6 /7 /S 7S 2QZ/Z22324252627Z8 29303/ 3Z 33 34-3 5 36 37 3 8 39 40 41 42 4344 454647 
/>£XC£A/TAG£ Cf? C/ZOSS/A/G QV£/e, 
Fig. 1. — The percentage of crossing over in the male gametes of individual plants of 
Dh 416L3L1C1L2L23 plotted with a range of ± three times the probable error. 
The short vertical lines indicate the percentage of crossing over 
The behavior of the crossover ratio of four closely related 
progenies over a period of several generations is shown graphically 
in Figures 2, 3, and 4. 
Both selfed and back-crossed ears are included. The selfed ears 
are located at the observed rate. The observed rate in back-crossed 
ears was corrected for difference in sex by taking the geometric mean 
of the observed rate and the mean rate of the opposite sex for the 
progeny. 
The figures in the body of the diagram show the number of ears 
having the percentage of crossing over indicated by the position of 
the figure in each instance. 
All progenies are the result either of selfing or of back crosses with 
sibs. 
CROSSING OVER GREATER IN SOME PROGENIES THAN IN OTHERS 
Attempts have been made to arrive at a single expression for the 
distance separating two linked genes on the chromosome. In the 
light of the data accumulated for the C-~Wx genes such efforts 
19987°— 27- 
