7 6 



SEX-LINKED INHERITANCE IN DROSOPHILA. 



LETHAL Ib. 



A cross between yellow white males and abnormal abdomen females 

 gave (February 1914) regular results in 10 F 2 cultures, but three 

 cultures gave 2 9 : i cf sex-ratios (Morgan, 1914^, p. 92). The yellow 

 white class, which was a non-cross-over class in these 10 cultures, 

 had disappeared in the 3 cultures. Subsequent work gave the data 

 summarized in table 57. At the time when the results of table 57 were 

 obtained it did not seem possible that two different lethals could be pres- 

 ent in the space of about I unit between yellow and white, and this lethal 

 was thought to be a reappearance of lethal I (Morgan, 191 2&, p. 92). 

 Since then a large number of lethals have arisen, one of them less than 

 o.i unit from yellow, and at least one other mutation has taken place 

 between yellow and white, so that the supposition is now rather that 

 the lethal in question was not lethal I. Indeed, the linkage data show 

 that this lethal, which may be called lethal ib, lies extraordinarily close 

 to white, for the distance from yellow was 0.8 unit and of white from 

 yellow on the basis of the same data 0.8. There was also a total 

 absence of cross-overs between lethal ib and white in the total of 846 flies 

 which could have shown such crossing-over. On the basis of this 

 linkage data alone we should be obliged to locate lethal ib at the point 

 at which white itself is situated, namely, i.i, but on a priori grounds it 

 seems improbable that a lethal mutation has occurred at the same locus 

 as the factor for white eye-color. Further evidence against this sup- 

 position is that females that have one X chromosome with both yellow 

 and white and the other X chromosome with yellow, lethal, and white 

 are exactly like regular stock yellow white flies. The lethal must have 

 appeared in a chromosome which was already carrying white and yet 

 did not affect the character of the white. We prefer, therefore, to 

 locate lethal ib at i.i . 



TABLE 57. Summary of all linkage data upon lethal ib, from Morgan, 



FACET. 



Several autosomal mutations had been found in which the facets of 

 the compound eye are disarranged. One that was sex-linked appeared 

 in February 1914. Under the low power of the binocular microscope 

 the facets are seen to be irregular in arrangement, instead of being 

 arranged in a strictly regular pattern. The ommatidia are more nearly 

 circular than hexagonal in outline, and are variable in size, some being 

 considerably larger than normal. The large ones are also darker than 



