NEW DATA. 



75 



Lethal 3 proved to lie between club and vermilion, 13 units from 

 club and 5 from vermilion. The same locus was indicated by the data 

 from the cross of vermilion lethal-bearing females by eosin miniature 

 males. The complete data bearing on the position of lethal 3 is sum- 

 marized in table 56. On the basis of this data lethal 3 is located at 26.5. 



Table 56. — Summary of linkage data on lethal 3, from Morgan, igi^. 



LETHAL 3a. 

 In January 1914 a vermilion female from a lethal 3 culture when 

 bred to a vermilion male gave 71 daughters and only 3 sons; 34 of these 

 daughters were tested, and every one of them gave a 2 : i sex-ratio. 

 The explanation advanced (Morgan 1914c) was that the mother of the 

 high ratio was heterozygous for lethal 3, and also for another lethal 

 that had arisen by mutation in the X chromosome brought in by the 

 sperm. On this interpretation the few males that survived were those 

 that had arisen through crossing-over. The rarity of the sons shows 

 that the two lethals were in loci near together, although here of course 

 in different chromosomes, except when one of them crossed over to the 

 other. As explained in the section on lethal i and la the distance 

 between the two lethals can be found by taking twice the number of the 

 surviving males (2-f 3) as the cross-overs and the number of the females 

 as the non-cross-overs. But the 34 daughters tested were also non- 

 cross-overs, since none of them failed to carry a lethal. The fractions 



; = give ?.? as the distance between the lethals in question. 



71 + 34 105^ ^' /* 



In the case of lethals 3 and 3a another test was applied which showed 



graphically that two lethals were present. Each of the daughters 



tested showed, by the classes of her sons, the amount of crossing-over 



between white and that lethal of the two that she carried. These 



cross-over values were plotted and gave a bimodal curve with modes 7 



units apart. It had already been shown that the locus of one of the 



two lethals was at 26.5, and since the higher of the two modes was at 



about 23, it corresponds to lethal 3. The data and the curve show 



that the lethals 3 and ^a are about 7 units apart, i. e., lethal 3 lies 



at about 19.5. 



