30 AN ANALYSIS OF THE EFFECT OF SELECTION. 
constitution, but there are also the crossover eggs, D’ l,, and +. 
If we let the non-crossovers be to the crossovers as x: y, the result of 
the mating will be: 
(D’ (Dl 
a =dies am = di 
Dp D'tun _ dies 
jlo y Jun 
3 TIT D’ cn as D’ 
aia D’ 
boot dies = not-D’ 
Can lu 
The result then is: 
2a+y=D’ y=not-D’ 
100y _ 200 (not-D’) 
x+y D’+not-D’ 
Per cent crossovers = 
In the present case this formula gives the crossover percentage 
as 0.29. Lethal III is, then, located 0.29 to the right of Dichet. 
Another lethal of the same sort as the one just described appeared 
in culture 1546. This culture belonged to the sixth generation of the 
same line in which the first lethal was found, and was descended from 
a sister pair (1213) to 1264, the first culture in which that lethal ap- 
peared. Since the two lethals are certainly distinct, as will appear 
below, this relationship is to be regarded only as a coincidence. Three 
cultures of this strain were made—1546 and two daughter pairs. 
The result was 154 Dichets and 1 not-Dichet. The 1 not-Dichet 
was from culture 1681. The Dichets from this culture show both 
parents to have had the constitution 
D' 
SelM 3,57 
The not-Dichet individual was spineless, sooty, rough. This indi- 
cates that the lethal was to the left of Dichet; otherwise the egg in 
question must have resulted from a sepia Dichet spineless triple cross- 
over, which is a very rare occurrence. By the method outlined above 
it may be calculated that the lethal gives 1.29 per cent of crossovers 
with Dichet. 
That these two lethals are distinct is indicated by the following 
culture, 1915. The female of this mating came from culture 1791, 
which gave 31 Dichets and no not-Dichets. 1791 was an F, from a 
cross involving 1419 of the 1264 line, and thus its lethal must be sup- 
posed to be that of 1264. The male of the test bottle 1915 was from 
1681 of the second lethal strain. Therefore, if the two lethals are the 
same, 1915 should have given few or no not-Dichets; if they are dif- 
ferent it should have given 2 Dichzts to 1 not-Dichet. The actual 
