344 
Journal of Agricultural Research voi. xxix, no. 7 
Table VI gives the frequency dis¬ 
tribution of F 3 head rows by date of 
first heading of the F 2 parental segre¬ 
gates and by time-temperature effi¬ 
ciency units required to bring F 3 head 
rows from date of first heading to com¬ 
plete heading. Figure 4 gives a dis¬ 
tribution curve of these data. If the 
one-factor hypothesis for earliness is 
correct, there should be an equal num¬ 
ber of homozygous and heterozygous 
rows in the F 3 progeny, according to the 
symbols AA+2Aa+aa, where A equals 
the factor for earliness and a that for 
lateness. The total number of indi¬ 
viduals observed in this group was 791. 
Whenan arbitrary division at the mini¬ 
progeny which were apparently fully 
homozygous in the F 3 . Also 293, or 
48.2 per cent, from a total of 609 plants 
classed as early in the F 2 , fall into the 
heterozygous class. This is nearly a 
1 : 1 ratio, whereas that expected is 2 :1. 
The deviation from the expected ratio 
may indicate that the division between 
homozygous and heterozygous head 
rows has been placed too high. Fur¬ 
thermore, it may indicate some short¬ 
coming in the method of calculating 
time - temperature efficiency units. 
Another possible cause is the difficulty 
experienced in determining accurately 
the end point for heading when taking 
notes. 
Fig. 4. —Curve showing frequency distribution of Fj head rows by date of first heading of the Fa parental 
segregates and by time-temperature efficiency units required to bring F 3 rows of early and late segre¬ 
gates from the F 2 progeny from first heading to complete heading 
mum frequency class of 1,251-1,300 
time-temperature efficiency units is 
made and this class included with the 
heterozygotes, 481 rows, or 60.70 per 
cent, were homozygous and 39.30 per 
cent heterozygous, or a ratio of 
1.21 :0.79, which indicates the ex¬ 
pected 1 : 1 ratio. By shifting the 
division slightly to the left this ratio can 
be obtained almost exactly. 
When the division is made at the 
1,251-1,300 class, 21 progeny rows of 
the late segregates in the F 2 , or about 
11 per cent, are classed as heterozygous. 
In addition, certain F 2 plants which 
began to head about midseason gave 
Table VII gives the frequency dis¬ 
tribution of the F 3 head rows by date of 
first heading of these rows and by total 
time-temperature efficiency units re¬ 
quired to bring each head row from the 
date of first heading to the date of com¬ 
plete heading. This table shows a defi¬ 
nite segregation into a homozygous late 
group. Of the total of 819 individuals 
(including 28 hybrid plants of this cross 
not tagged for first heading in F 2 ), 193, 
or 23.41 per cent, fall into this group. 
For the whole population this makes a 
ratio of 3.08 : 0.92, which agrees closely 
to a 3 : 1 ratio. The total number of 
individuals in the early group was 627. 
