SEX-UNKED LETHALS IN DROSOPHILA 121 



Rawls" 



In those cases where the offspring were tested (Rawls) it was 

 found that the very high ratio disappeared. It seems to be due 

 to a particular combination that was subsequently lost. If we 

 assume that two linked lethals occurred, one in each of the sex 

 chromosomes of the mothers that gave these high ratios, an 

 explanation of the results is apparent. Such a female herself 

 could live, since the lethals affecting different parts of the indi- 

 vidual have each its normal allelomorph in the other sex chromo- 

 some. The eggs produced would be in the main of two kinds — 

 half containing one lethal, half the other. Since all of the sons 

 derive their single sex-chromosome from the mother, they will 

 perish. But if crossing over in sonie of the eggs of the Fi female 

 should occur then in such eggs one of the chromosomes will get 

 both lethals, and the other both the normal allelomorphs. The 

 former type of egg, if fertilized by a male-producing sperm, 

 would fail to produce a male; the latter type oi egg would pro- 

 duce a normal male. The few males that appear may repre- 

 sent this class. 



There are two ways in which the original double lethal female 

 might be imagined to arise. A new sex-linked lethal might 

 appear in one of the sperm-cells. If such a spermatozoon fer- 

 tilizes an egg already containing a lethal a double lethal female 

 that is viable would result. The frequency with which lethal 

 mutants have turned up would make plausible this assumption. 

 The same end would be reached if, in a female already contain- 

 ing one sex-linked lethal, another lethal should appear in the 

 other sex-chromosome. 



For two other results no hypothesis even can be suggested. 

 First, the remarkable sterility found in the Fi males when these 

 high sex-ratios appeared; second, the converse case described by 

 Quackenbush where 135 males and no females appeared. 



* Rawls, E. Sex ratios in Drosophila ampelophila. Biol. Bull., vol. 24, Janu- 

 ary, 1913. 



