256 PHYSICAL BASIS OF HEREDITY 



stage. Other related (wild) species and genera of the 

 evening primrose have also been found to have some 

 abortive pollen and ovules. 



Complete or nearly complete abortion has been seen 

 in other hybrids; viz., by Rosenberg in the sundew, by 

 Osawa in the Satsuma orange, by Goodspeed and others 

 in the hybrid tobacco {N. tabacum by N. sylvestris), by 

 Jesenko in the wheat-rye hybrid, and by Sutton in the 

 hybrid between the Palestine pea {Pisum humile) and the 

 edible pea. These cases may be in part the same phe- 

 nomenon and in part a different one comiected with fail- 

 ure of the chromosome to conjugate or to be properly 

 distributed during the maturation divisions. 



The ^* yellow mouse case'' is an example of a zygotic 

 lethal effect. The gene that produces the dominant yellow 

 color is lethal in double dose, so that all homozygous yel- 

 low mice die, as Cuenot first discovered, and as has been 

 more positively demonstrated by the work of Castle and 

 Little. There is some evidence indicating that these homo- 

 zygotes die as young embryos. Little has also shown that 

 black-eyed white mice carry a lethal, that acts in the same 

 way. In Drosophila there is a sex-linked recessive lethal 

 factor that causes the development of tumors in the larvae, 

 destroying every male larva that contains the sex-chromo- 

 some carrying this gene. This effect, discovered by 

 Bridges, has been the basis for an extensive series of 

 experiments by Miss Stark. The gene is present in the 

 X-chromosomes ; it follows the rules for all sex-linked 

 genes in its inheritance. The females of the stock are of 

 two kinds : One has the lethal in one sex-chromosome, and 

 its normal, dominant allelomorph in the other. Such a 

 female has survived because the effect of the lethal gene 

 is counteracted by the effect of its normal allelomorph. 

 Half of her sons get the affected chromosome. All such 

 sons develop the tumor — one or more melanitic growths 

 that appear in the imaginal discs or in other parts of the 

 larva. The other sons get the other chromosome with the 



