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INTRODUCTION TO CYTOLOGY 



The form of human colorblindness known as Daltonism, in which green 

 and red are not properly distinguishable, is a sex-linked character which 

 is inherited precisely like white eyes in Drosophila. The presence of 

 this defect more commonly in men than in women, and its appearance in 

 so few individuals in affected lines, are due to the fact that it is a recessive 

 and sex-linked character. It occurs in a woman only if both of her 

 A^-chromosomes bear factors for it, which means that one such factor 

 must have been received from each parent; whereas one factor is sufficient 



DTIOSQPHILA 





0(9 ) (60 



Fig. 225. — Sex-linkage in Drosophila. Three successive generations at left; red eyes 

 shown in black. History of sex-chromosomes through these generations shown at right, 

 the X-chromosome of the original male in black. {Adapted from Morgan.) 



to produce it in a man, because his F-chromosome carries no factors which 

 might dominate it. Furthermore, since the X-chromosome of the male is 

 always derived from the mother, a man can inherit colorblindness from his 

 mother but not from his father. He might appear to do so, however, if 

 his father were colorblind and his mother apparently normal but carrying 

 one affected X-chromosome: the defect in this case would actually be 

 transmitted by the mother, though apparently by the father. In general, 

 therefore, a colorblind woman transmits the defect to all of her sons and 

 to half of her grandsons and granddaughters, whereas a colorblind man 

 transmits it to none of his children and only to one half of his grandsons. 

 Haemophilia is inherited in a similar manner.^'' 



Sex-linkage in plants was first observed in Melandrium {Lychnis).^'' 

 The genetic data indicate that the genes for "angustifolia" and "aurea" 



^^ For literature on sex-linkage in man, see Davenport (1930). 

 3^ Baur (1912), G. H. Shull (1914), Winge (1927c, 1930c, 1931). 



