INTRODUCTORY. I5 



such a condition, do not exist. The presumption is that a female of 

 this kind would also die if the lethal acts injuriously on some vital 

 function or structure. 



Since only half of the daughters of the lethal-bearing females carry 

 the lethal, the stock can be maintained by breeding daughters separately 

 in each generation to insure obtaining one which repeats the 2 : i ratio. 

 There is, however, a much more advantageous way of carrying on the 

 stock — one that also confirms the sufficiency of the theory. 



In carrying on a stock of a lethal, advantage can be taken of linkage. 

 A lethal factor has a definite locus in the chromosome; if, then, a 

 lethal-bearing female is crossed to a male of another stock with a reces- 

 sive character whose factor lies in the X chromosome very close to 

 the lethal factor, half the daughters will have lethal in one X and the 

 recessive in the other. The lethal-bearing females can be picked out 

 from their sisters by the fact that they give a 2 : i sex-ratio, and by the 

 fact that nearly all the sons that do survive showthe recessive character. 

 If such females are tested by breeding to the recessive males, then the 

 daughters which do not show the recessive carry the lethal, except in 

 the few cases of crossing-over. Thus in each generation the normal 

 females are crossed to the recessive males with the assurance that the 

 lethal will not be lost. If instead of the single recessive used in this 

 fashion, a double recessive of such a sort that one recessive lies on each 

 side of the lethal is used, then in each generation the females which 

 show neither recessive will almost invariably contain the lethal, since 

 a double cross-over is required to remove the lethal. 



It is true that females carrying two different lethals might arise and 

 not die, because the injurious effect of each lethal would be dominated 

 by its allelomorph in the other X chromosome. Such females can not 

 be obtained by combining two existing lethals, since lethal males do 

 not survive. They can occur only through a new lethal arising through 

 mutation in the homologous chromosome of a female that already 

 carries one lethal. Rare as such an event must be, it has occurred in 

 our cultures thrice. The presence of a female of this kind will be at 

 once noticed by the fact that she produces no sons, or very rarely one, 

 giving in consequence extraordinary sex-ratios. The rare appearance 

 of a son from such a female can be accounted for in the following way: 

 If crossing-over occurs between her X chromosomes the result will be 

 that one X will sometimes contain two lethals, the other none. The 

 latter, if it passes into a male, will lead to the development of a normal 

 individual. The number of such males depends on the distance apart 

 of the two lethals in the chromosome. There is a crucial test ot this 

 hypothesis of two lethals in females giving extraordinary ratios. This 

 test has been applied to the cases in which such females were found, 

 by Rawls (1913), by Morgan (i9i4<:), and again by Stark (191 5), 

 and it has been found to confirm the explanation. The daughters of 



