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 /. 

 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 I. 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 L 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 Jf-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 L 3 . These females, 

 homozygous for L 3 , produce flies in the ratio of 1 red 9 : 1 red cT : 1 white 

 9 : 1 white cf when mated to their brothers. The other half of the Fi 

 females, on the other hand, will be heterozygous for L 3 and conse- 

 quently, since crossing-over takes place in 25 per cent, of cases, they 

 produce gametes in the ratio 3(wl 3 X):3(WL 3 X):l(Wl 3 X):l(wL 3 X). 

 When such a female is mated to an Fi male fly the ratio is distinctly 

 different from that obtained with the other females, in this case 4 Red 

 9 :3 Red cf:4 White 9 :1 White cf . The ratio of sexes in this latter 

 case is 2 female : 1 male and the same is true in FI. 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 



