434 



THE I'OPTLAi; SCIENCE MONTH LV 



all probability associated with tbe distribution of the maternal and 

 paternal sex chromosomes. 



One of the most striking cases of sex-linked inheritance is that form 

 of color-blindness known as Daltonism, in which the affected person is 

 unable to distinguish between red and green. It is known that males 

 are more frequently affected tban females, and that color-blindness is 

 in some way associated with sex. It requires two determiners for color- 

 blindness, one from tbe father, the other from the mother, to produce a 

 color-blind female, whereas only a single determiner is necessary to 



Flies 



Chromosomes 



Gametes 



9 9 6 



Fig. 62. Reciprocal, Cross of Fig. 61. Parents, white-eyed <J> and red-eyed J 1 , 

 F lt red^eyed J and white-eyed g ("Criss-cross inheritance"), F 2 , equal numbers of 

 red-eyed J an< i $ and white-eyed J and J". The distribution of sex chromosomes is 

 shown on the right, as in Fig. 61. 



produce a color-blind male, just as is true of sex. The accompanying 

 diagrams illustrate the method of inheritance of color-blindness. As in 

 the previous diagrams X represents the sex determiner, its absence, and 

 X the sex determiner which carries tbe factor for color-blindness. (Dia- 

 grams from Morgan.) It will be seen that a color-blind father and a 

 normal mother have only normal children, but the father transmits to 

 I lis daughters and not to his sons the sex determiner which carries the 

 factor for color-blindness. But since color-blindness does not develop 

 in females unless it is duplex (/. c, comes from both father and mother), 

 whereas it develops in males if it is simplex (i. e., comes from either 

 parent) all the daughters will appear normal although carrying one 

 determiner for color-blindness, while all the sons will be normal because 

 they carry no determiner for color-blindness. But these daughters trans- 

 mit to one half of their children tbe single determiner for color-blind- 

 ness,- and if any of those receiving this determiner are males they will 



