MENDELIAN SEX-FACTORS IN MAN. 

 By J. A. JENKINS. 



That the numerical equality of the sexes in so many types may be 

 most simply explained on the assumption that one sex is homozygous, 

 and the other sex heterozygous for sex-factors ("maleness" and "female- 

 ness"), is I believe commonly granted. Again, several instances are 

 known in which there appears to be (during gametogenesis) "repulsion" 

 between a sex-factor and a character-unit, resulting in a " sex-limited " 

 descent of that character ; but no clear case of " coupling " in similar 

 circumstances has yet been brought forward. A necessary condition for 

 either repulsion or coupling to take place is the heterozygosis of both 

 factors in the parent concerned (Bateson, Mendel's Principles of 

 Heredity, C. U. Press, p. 151). In sex-limited cases the following 

 general considerations arise : 



(1) As by our hypothesis one sex is homozygous and the other 

 heterozygous, the dissimilar sox-factor borne by the latter parent must 

 in all cases be dominant : also, it is in the gametogenesis of this parent 

 that we look for the "sex-limitation." 



(2) The process in such cases being due to repulsion existing 

 between dominant factors, we expect to find among the progeny that 

 individuals heterozygous in sex bear the recessive character (of another 

 pair of factors) more frequently than individuals homozygous in sex. 



Applying these rules to human colour-blindness, and taking colour- 

 blindness, n, to be recessive to normal vision, N (as shown to be the 

 case by Doncaster, Journal of Genetics, Vol. I. p. 378), we see that 

 probably in Man, the female is homozygous and the male heterozygous 



