378 



Inheritance of Characters 



The explanation commonly given of these facts has been that 

 colour-blindness is dominant in the male, recessive in the female, so 

 that the male heterozygote is colour-blind, the female heterozygote 

 normal ; a colour-blind woman can thus arise only when the affection 

 is inherited from both parents. It is also evident that the affected 

 male transmits the factor for the disease only to his daughters ; the 

 heterozygous female, however, transmits to some of the offspring of 

 both sexes. This sex-limitation of the transmission makes a diffei'ent 

 explanation possible, which is also more in accord with other cases 

 of sex-limited inheritance. 



Since the male transmits the factor for colour-blindness only to 

 his daughters, it must be assumed that the male in this case is 

 heterozygous for the sex-determinei\ In former papers I have sug- 

 gested that if maleness is determined by a factor J*, femaleness by a 

 factor % epistatic to ^ when both are present, then a male individual 

 may be represented cfO, a female </" %; i.e. that both sexes are hetero- 

 zygous for sex-determiners, with selective fertilisation between j/' -bearing 

 eggs and 0-bearing spermatozoa, and between $ -bearing eggs and </- 

 bearing spermatozoa'. If we adopt this scheme as a working hypothesis, 

 and then represent normal sight by N, colour-blindness by absence or 

 modification of iV (=?(), and further suppose that N can only be borne 

 by gametes containing a sex-determiner (</ or $ , not 0), we obtain the 

 observed results. 



1 1 



n <f N S N s ni N s Ni 



(affected male) (normal male) (normal female (normal female) 

 I heterozygous) 



gametes 



ni 

 (affected male) 



ns , O 



N i n i 

 (heterozygous 

 female) 

 N i , n i 

 ns , N i 



I 1 



ni N i O mm ni N i 



(affected male) (normal male) (affected female) (normal female 



heterozygous) 



' Proc. Roy. Soc. B. 82. 1910, p. 88, B. 83. 1911, p. 476. My reasons for continuing to 

 prefer this scheme to that of the American writers, who represent the male as XO, the 

 female A'.Y, will be given in a subsequent paper on the same subject. The argument in 

 the present case applies equally to both schemes. 



