Inheritance of Salmon Silk Color in Maize 
553 
It should be understood that the chromosome map is primarily a graphic 
representation of the data on linkage relationships. Its correspondence 
with actual positions on the chromosome itself is not implied, tho the 
work of Morgan and his coworkers has given much evidence of at least a 
correspondence between relative map order and the actual relative position 
of the genes in the chromosome. 
The variability of the percentages of crossing-over shown in table 6 
is not greater than would be expected of heterogeneous data. Gowen 
(1919) has shown crossing-over in Drosophila to be an extremely variable 
phenomenon. Plough (1917) has shown it to be modified by temperature, 
and Bridges (1915) by age of the individual. The subject of variation 
of crossing-over in maize must remain for study with less difficult characters 
than those involved in these experiments. 
The distributions when the Fi's were used as pistillate and as staminate 
parents give nearly the same averages, but the data are inadequate for 
any conclusion except that the crossing-over is not widely different in 
the two cases. 
Coincidence of crossing-over in two regions of a chromosome is the 
ratio of observed coincident (simultaneous) crossing-over to the calculated 
expectancy. The expectancy is the product of the percentages of crossing- 
over of the two regions. The actual calculation may be simplified, as 
shown by Weinstein (1918). The derived formula is 
in which n = the total number of individuals, 
x = the number of coincident crossovers, 
a and b = the total number of crossovers in the respective regions. 
The coincidence of crossing-over in the two regions Y-Pl and Pl-Sm, 
calculated from tables 4 and 5, is as follows: 
From table 4 : 
Coincidence of crossing-over 
xn 
Coincidence = 
21 x 3093 
893 x 281 
-0.26 
From table 5: 
Coincidence — 
24 x 930 
302 x 94 
= 0.79 
