H. Onslow 137 



suggests itself with the inheritance of "notch" wing in Drosophila\ This 

 variation is dominant and sex-linked to the female ; associated with the 

 homozygous condition is a lethal factor, causing homozygous females to 

 die, but the heterozygous females live. Moreover, all males carrying 

 " notch " die, because the Y chromosome is supposed to be empty, and 

 therefore the insects do not possess the normal allelomorph for " notch " 

 which, in the heterozygous females, counteracts the lethal factor. If in 

 the case of radiation, conditions were the same, except that the factor 

 was linked to the male sex, then varleyata % % (het. for radiation) x 

 varleyata (/(/ should give equal numbers of varleyata cfcT and $ $ , and 

 the same number of radiated c/cT, but no radiated ? %. 



When more data are available, it will be easier to decide whether a 

 lethal factor is present or not ; also whether " notch " wing, colour- 

 blindness, or some other mode of inheritance is the most correct type 

 on which to base the explanation of the anomalous sex-ratio observed. 

 At present only some of the possibilities can be indicated, but it should 

 be remembered that, whatever explanation is adopted, it must take into 

 account the fact that at least one radiated female has been produced. 

 Moreover, attention may be called to the sex ratio in the varleyata x F^ 

 families (see p. 126) which show a considerable excess of males in both 

 black and type insects, no matter whether the radiated families, marked 

 with an asterisk, are included or not. 



If the factor for radiation is supposed to be recessive, and sex-linked 

 to the male, Professor Punnett has raised an interesting question. 

 Supposing the missing chromosome observed by Doncaster in the all- 

 female strains of grossulariata to be a sex chromosome, then according 

 to the chromosome hypothesis, the factor for radiation Avould be carried 

 by the Z chromosome. A female, heterozygous for radiation, would have 

 that factor in her Z chromosome. Thus if Morgan's suggestion that the 

 W chromosome is a "dummy" without factors, is accepted, then all such 

 heterozygous females must be radiated in appearance. To avoid this 

 difficulty, either the W chromosome must carry the normal allelomorph 

 which prevents the development of the white radiation, or the W 

 chromosome itself must be able to inhibit radiation in a female that is 

 genetically actinota, which is after all much the same thing. From his 

 work on poultry. Professor Punnett- is inclined to believe that if sex 

 chromosomes exist in these cases, they can carry factor.s. 



1 Morgan, T. H., Publications of the Carnegie Institute, No. 343, 1919. 

 ■^ Punnett, E. C, Journal of Genetics, Vol. xi. No. 1, p. 37, April, 1921. 



