I 12 



HEREDITY AND EUGENICS 



An early record of sex-linked colour-blindness 

 (*' Daltonism ") is of such interest that it is included 

 here. It was communicated to the Royal Society in 

 1 778 by the Rev. Michael Lort, and consists of a letter 

 from Mr. J. Scott to the Rev. Mr. Whisson of Trinity 

 College, Cambridge, describing his infirmity, and 

 stating which of his relatives also possessed it. This 

 has been thrown into the form of a pedigree chart 

 by Cole (191 9), from whom Fig. 20 is taken with 

 modifications. No. III. 3 is the deponent, and it 



n 



in 



l^-^ 



L_J 



XYorXY 



ffi 



4 



XX 



k^ 



/,— . 



2,-s 



XYorXY 



XXorXX 



XY 



XY 



4.- 



XYorXY 



o 



XX 



4 



o 



XXorXX 



1 



XY 



XX 



XY XY XY XX 



Fig. 20. — Pedigree Chart of Colour-Blindness. 



will be seen that his father and his mother's brother 

 were also colour-blind. His mother, though normal, 

 had a colour-blind son and daughter, and she must 

 therefore have been heterozygous for the character 

 (see Fig. 8, p. 23), which gives the history of the sex 

 chromosomes. Homozygous colour-blind mothers 

 transmit the defect to all their sons, as in generation 

 IV., while normal but heterozygous mothers transmit 

 it to half their sons. 



Fathers, on the other hand, never transmit the 



