26 Congenital Night-Blind7iess 



The association of families dealt with in this paper is confined to 

 the State of Texas, but originated in the State of North Carolina. 

 Whether or not the strain can be traced back to an English origin I am 

 unable to ascertain. It would be interesting to find that the present 

 line is a branch of one of those investigated by Mr Nettleship. In order 

 that it may be made possible to trace such connection, if it exist, I shall 

 give the full names, where obtainable, of ail individuals concerned. 



Especial interest attaches to the present pedigree first because it 

 appears to follow a formula hitherto undescribed for night-blindness 

 and second because the major defect is found so generally associated 

 with much less rare disease of the cornea known as Pterygium, which 

 is not sex-limited but more pronounced in males than in females. 

 Strabismic and myopic conditions appear to be in some way tied up 

 with night-blindness and are also sex-limited. 



The peculiarities in the descent of the various sex-limited characters 

 of man are given by Bateson as follows : 



1. They affect males much more commonly than females. 



2. They may in certain cases be transmitted by affected males, but 

 are not transmitted by unaffected males. 



3. They are nevertheless transmitted by the unaffected females. 

 Apparently normal women, sisters of the affected males, thus may 

 transmit the condition to some of their sons. 



The case herewith described falls in line with none of these 

 schemes of inheritance but would demand a fourth formula somewhat 

 as follows : 



4. They are not transmitted directly hy males, either affected or 

 unaffected, hut only hy the unaffected daughters of affected males to some 

 of their sons. 



That this method of inheritance was recognized by Bateson but 

 inadvertently omitted is shown in a note facing page 230 of his book, in 

 which he corrects an error regarding the inheritance of colour-blindness. 

 He says : 



" In haemophilia and night-blindness there are cases of the direct 

 descent of the condition from father to son in families showing ordinary 

 sex-limited descent, but as regards colour-blindness, though there are 

 instances of the direct descent from father to son, it is now obvious that 

 in all of them the affection was introduced by the normal-sighted 

 mother also. Apart from these there is no case of a colour-blind man 

 having a colour-blind son known to me. On the contrary, from the 

 records, for the most part communicated by Mr Nettleship, there is 



