SEX-CHARACTERS IN EVOLUTION 113 



of the white-eyed male. The gametes of the female 

 each carry one X red chromosome, of those of the 

 male half carry an X white chromosome, and half 

 the Y white chromosome. The fertilised female ova 

 therefore carry an X red chromosome + an X white 

 chromosome, the male producing ova one X red 

 chromosome and one Y white chromosome. They 

 are all therefore red-eyed, but heterozygous — that 

 is, the red eye is due to one red-eye factor, not two. 

 When the F^ are bred together, half the female 

 gametes carry one X red chromosome, the other half 

 one X white chromosome ; half the male gametes 

 carry one X red chromosome, the other half one Y 

 white chromosome. The fertilisations are therefore 

 one X red X red, one X red X white, one X red Y 

 white, and one X white Y white. These last are the 

 white-eyed males. The two different crosses are 

 represented diagrammatically on p. 114, the dark 

 rod representing the X red chromosome, the clear 

 rod the X white chromosome, and the bent clear 

 rod the Y white chromosome. 



According to Morgan, the heredity of colour- 

 blindness in man is to be explained exactly in the 

 same way as that of white eye in Droso2)hiIa. A 

 colour-blind man married to a normal (homozygous) 

 woman transmits the peculiarity to half his grand- 

 sons and to none of his grand-daughters. Colour- 

 blind women are rare, but in the few cases known 

 where such women have married normal husbands 

 the defect has appeared only in tlie sons, as in the 

 second of the diagrams on p. 114. It must be explaiiuMi 

 that according to this theory the normal male is 

 always heterozygous, because the Y chromosome 

 never carries any other factor except that for sex ; 



H 



