THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 133 
factors in Drosophila, it follows that the strains must be propagated by 
means of females heterozygous for the factors. The diagram shows 
how such lines are maintained. A heterozygous female produces eggs 
half of which bear the normal factor L, and half bear the lethal factor 1. 
When mated to a normal male the X-chromosome of which bears the 
normal factor L, half the daughters are normal homozygotes and half 
are heterozygous for J. Half the males receive an X-chromosome bear- 
ing the factor L, and consequently are normal and half receive an X- 
chromosome bearing the factor 1. These latter die before reaching the 
adult stage, consequently a heterozygous female produces flies two- 
thirds of which are females and one-third males The unusual sex- 
ratio provides a convenient test for heterozygous females and by this 
means the strain may be continued. 
Some of the consequences of the presence of lethal factors when 
linked with other factors are of importance because of the disturbances 
to which they give rise in Mendelian experiments. An illustration of 
such effects may be taken from Lethal III in Drosophila which is located 
at about the locus 26.5 in the X-chromosome. It is about 25 units 
distance from the locus for white eyes. If now a white-eyed female 
heterozygous for Lethal III be crossed with a red-eyed male, as shown 
diagrammatically in Fig. 58 all the females will be red-eyed but only 
“half will be homozygous for the normal factor L3;. These females, 
homozygous for Ls, produce flies in the ratio of 1 red 9:1 red @:1 white 
@:1 white o& when mated to their brothers. The other half of the F; 
females, on the other hand, will be heterozygous for L3 and conse- 
quently, since crossing-over takes place in 25 per cent. of cases, they 
produce gametes in the ratio 3(wl3X):3(WD3X):1(W13X):1(wL3X). 
When such a female is mated to an F male fly the ratio is distinctly 
different from that obtained with the other females, in this case 4 Red 
9:38 Red o1:4 White 9:1 White o. The ratio of sexes in this latter 
case is 2 female:1 male and the same is true in F;. The sex ratio gives 
an immediate clue to the disturbing factor and leads to a true explanation 
of the cause of the disturbance. 
Manifold Effects of Factors.—In a preceding section of this chapter it 
has been shown how far reaching may be the effects of single Mendelian 
factors, and in the present account it is intended to deal specifically 
with what Morgan has termed the manifold effects of single factors. 
Careful study has revealed the fact that although factors are restricted 
in their conspicuous results to certain characters, nevertheless they 
may have other less noticeable results which are none the less definite 
and constant. Baur has observed for example in Antirrhinum that 
the factor which produces pure white blossoms also yields plants which 
are distinctly weaker in growth and are smaller than those which possess 
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