320 



Appejtdixes 



cannot transmit it. Doncaster shows that by assuming 

 both sexes to be heterozygous for sex-determiners it is 

 possible to construct a scheme on which normal vision is 

 the dominant, and colour-blindness the recessive, which 

 will express all the facts, and is in my judgment on the 

 whole the most probable account of these phenomena yet 

 suggested. Substituting white eye for colour-blindness, 

 the same scheme expresses Morgan's observations for red 

 and white eye in Drosophila. 



Doncaster's scheme is thus expressed : '' Since the 

 male transmits the factor for colour-blindness only to his 

 dauohters, it must be assumed that the male in this case 

 is heterozygous for the sex-determiner. In former papers 

 I have suggested that if maleness is determined by a 

 factor ^, femaleness by a factor % epistatic to ^ when both 

 are present, then a male individual may be represented ^ O, 

 a female ^ $ ; i.e. that both sexes are heterozygous for sex- 

 determiners, with selective fertilisation between ^-bearing 

 eggs and 6^-bearing spermatozoa, and between ^-bearing 

 eggs and ^-bearing spermatozoa. If we adopt this scheme 



gametes 



Parents 

 gametes 



gametes 



n$ O 



(affected male) 



v 



N$ O 

 (normal male) 



N6 iV? 

 (normal female) 



n 



(normal female 

 heterozygous) 



n$0 

 (affected male) 



n$ O 

 (affected male) 



n$,0 



N$ O 

 (normal male) 



(normal female (normal female) 

 heterozygous) 



JVS n ? 



(heterozygous 



female) 



n6 O N$ O 



(affected male) (normal male) 



(affected female) (normal female 



heterozygous) 



* Doncaster, ibid. p. 378. 



