SEX-LINKED LETHALS IN DROSOPHILA 117 



SEX-RATIOR HIGHER THAN 2:1 



A glance at the last columns of the tables will show in many 

 cases that the sex-ratio is higher than 2:1. Most of these differ- 

 ences are obviously chance deviations. There are some, how- 

 ever, that are so high as to suggest higher ratios. In fact, the 

 start was made with a 5:1 ratio. It seemed at first as though 

 those ratios higher than 2:1 might be due to two lethal factors 

 that were simultaneously present, although this involves either 

 two simultaneous mutations or the appearance of a new lethal in 

 an egg that already contained one (through mutation or fertili- 

 lization) . Whether such an hypothesis is improbable will depend 

 on how often lethal factors appear in stocks like these. Let us 

 see how it might be shown that the high ratios could be explained 

 as due to two lethals. If a third lethal I3 were present it would 

 be in all probability beyond miniature, or, like the second lethal, 

 between miniature and white. On the first supposition the pos- 

 sibilities are shown in figure F. There are three single cross- 

 over classes, three double cross-over classes and one triple cross- 

 over. Of the sixteen possible classes of males only four would 

 come through, as shown in the following list: 



, . -, f W I2 M I3 — not realized 



(a) J\ on-cross-over ^ p t t • • • i 



(w^ L2 m L3 — eosin miniature 



,,.„., fW L2 m L; — red miniature 



(b) Single cross-over (wM^ M I3 -not realized 



,,_,., fVV I2 m L3 — not realized 



(c) bingle cross-over ^ p t tvt 1 ^ i- j 



* [w'^ L2 M I3 — not realized 



,,,„., f W I2 M L3 — not realized 



(d) bingle cross-over < „t 1 , i- . 



l^w'' L2 m I3 — not realized 



, , „ , , f W L2 M I3 — not realized 



(e) Double cross-over . 



w*^ I2 m L3 — not realized 



,., -rx , , f W L2 m I3 — not realized 



(f) Double cross-over { p,,tt x i- 1 



l^w^ lo M L3 — not realized 



, , „ , , fW I) m I3 — not realized 



(g) Double cross-over < „. ^,, . , 



l^w*^ Lo M L3 — eosin long 



... „. , /W L2 M L3 —red long 



(h) Iriple cross-over ■( « , , ^ i- 1 



(w" I2 m I3 — not realized 



One of the four that are possible is the triple cross-over or red 

 long. A triple cross-over is a very rare occurrence and would 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 17, NO. 1 



