110 ORIGIN OP SOMATIC 



According to Doncaster 1 sex-limited, or as it is now 

 proposed to call it sex-linked, transmission in this 

 case means that the female grossulariata transmits 

 the character to all her male offspring and to none of 

 the female, while a heterozygous male grossulariata 

 mated with lacticolor female transmits the character 

 equally to both sexes : that is to say, the heredity is 

 completely sex-limited in the female but not at all in 

 the male. This is evidence that the female produces 

 two kinds of eggs, one male producing and the other 

 female producing. 



With regard to the ordinary form of colour- 

 blindness, Bateson's first explanation was that it was 

 like the horns in the cross-bred sheep, dominant in 

 males, recessive in females. About 4 per cent, of 

 males in European countries are colour-blind, but 

 less than \ per cent, of females. Affected males 

 may transmit the defect to their sons but not to 

 their daughters : but daughters of affected persons 

 transmit the defect frequently to their sons. Bateson 

 gives 2 a scheme of the transmission, but corrects 

 this in a note stating that colour-blindness does not 

 descend from father to son, unless the defect was 

 introduced by the normal sighted mother also, i.e. 

 was carried by her as a recessive. The fact that 

 unaffected males do not transmit the defect shows, 

 according to Bateson, that it is due to the addition 

 of a factor to the normal, not to omission of a factor. 



According to later researches as quoted by 

 Doncaster, colour-blindness is due to the loss of some 

 factor which is present in the normal individual. 

 The normal male is heterozygous for this normal 

 factor. If we denote the presence of the normal 



1 Determination of Sex, Camb. Univ. Press, 1914. 



2 Menders Principles of Heredity, 1909. 



