PHYSICAL CHARACTERS IN MAN 113 



defect to their sons ; but half their daughters by 

 normal mothers will, though normal, be transmitters 

 to half their sons by a normal father. Heterozygous 

 mothers with a colour-blind father will have 50 per 

 cent, of colour-blind daughters (while all the daughters 

 will transmit the defect), and half the sons of such a 

 marriage will also be colour-blind. Regarding the 

 deponent's grandparents there is no record, but the 

 grandmothers must both have been either colour-blind 

 or heterozygous transmitters of the defect, while the 

 grandfathers may or may not have been colour- 

 blind. 



A shorter account of another case of colour-blindness 

 is given in a letter written in the previous year by 

 Mr. Joseph Huddart to the Rev. Joseph Priestley, 

 and published in the Phil. Trans., 1777. A shoe- 

 maker named Harris from Maryport, Cumberland, 

 was colour-blind, having discovered it at the age of 

 four years through being unable to distinguish a red 

 stocking from an ordinary (presumably black) one. 

 He also observed that " when young, other children 

 could discern cherries on a tree by some pretended 

 difference of colour, though he could only distinguish 

 them from the leaves by their difference of size and 

 shape." Colour-blindness consists essentially in the 

 failure to distinguish red from green. I'his man 

 " had two brothers in the same circumstances as to 

 sight; and two other brothers and sisters who, as well 

 as their parents, had nothing of this defect." Evi- 

 dently the mother was heterozygous, and transmitted 

 the defect to half her sons. According to Bateson 

 (1909), in European countries at least 4 per cent, of 

 the male population and less than 0-5 per cent, of the 

 females are colour-blind. 



Bowditch (1922) describes three families related 

 through marriage, all showing the usual type of 

 sex-linked inheritance. In one of the families the 



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