106 



Mr. L. Doncaster. 



[Sept. 21, 



are not only two kinds of eggs differing in their sex-determinants, but also 

 two kinds of spermatozoa. In colour-blindness, for example, the affection 

 is dominant in the male and recessive in the female ; so that heterozygous 

 males exhibit it, heterozygous females do not ; but affected men do not 

 transmit it to their sons. The sons of affected males are normal, but the 

 daughters of affected males transmit the disease to some of their sons and, 

 through some of the daughters, to their grandsons. It is clear, therefore, that 

 among the spermatozoa of an affected male, those only bear the determiner 

 for the disease which will give rise to female zygotes ; but among tlie ova of a 

 female heterozygous for the colour-blind condition, the colour-blind factor 

 is borne indiscriminately by about half the ova, whether male-producing or 

 female-producing. 



These different facts all fall into line if we assume that the female contains 

 both female and male sex-determinants, and produces equal numbers of ova 

 bearing each ; while the male contains no female determinant, but produces 

 two kinds of spermatozoa, one bearing the male determinant, the other being 

 without any determinant for either sex. Using the same symbols as before, 

 viz., ? and S for the female and male sex-'determinants respectively, and the 

 symbol 0 for absence of sex-determinant in a gamete, females would then 

 have the constitution ? S> and would produce ? and ^ eggs in equal 

 numbers, males would be (JO, and produce spermatozoa S and O in equal 

 numbers. In the insects with heterochromosomes, in the male the hetero- 

 ■chroniosome is regarded as bearing J* , its absence being represented by O ; 

 in the female the two heterochromosomes bear ? and respectively. If we 

 regard the sex-determinants as equivalent to Mendelian allelomorphs, they 

 must be considered as two pairs, each determinant being allelomorpliic with 

 its absence, i.e. ? with 0, and S with 0, but ? and are spuriously 

 allelomorpliic with each other, so that when they coexist in the same 

 zygote they cannot both enter the same gamete ; just as in tlie case of 

 Abraxas, the V determinant and the groamlariata determinant exhibit 

 spurious allelomorphism, and are never found in tlie same gamete. We 

 shall then have females producing equal numbers of ? eggs and ^ eggs, 

 males producing e([ual numbers of S and © spermatozoa, and it must 

 further be assumed that ? eggs are fertilised by S spermatozoa, giving 

 females eggs by 0 spermatozoa, giving males ((?0). 



This scheme is consistent with the facts in three categories of cases which 

 have hitherto seemed irreconcilable, viz., Abraans and tliose cases which 

 resemble it; tlie insects with heterochromosomes, and the cases of sex- 

 limited inheritance of which colour-blindness is the type. In Abraxas there 

 is spurious allelomorphism between the ? determinant and the grosmlariata 



