132 Proceedings of Boyal Society of Edinburgh. [sess. 
ness was the first noticed, when I was five years old ; hut my 
eldest brother is the worst case, and my third brother the mildest. 
Fm/rtli Aunt (bg) has two sons and two daughters ; the elder son, B^, 
colour-blind, and the rest free. 
The result is this, that the sons of the patient and their descendants are 
free. The daughters of the patient and their daughters and their descendants 
are also free. In fact, one son, or two female descents, break the spell. But 
where the daughter of a patient has sons, one of these at least is always a 
victim, and will transmit it to his daughters’ sons. 
I don’t know whether this is a general rule, but I may suggest that the fact 
of the change of surname, which such a descent entails, would operate much 
to conceal such a canon of descent from observation. People by their “ family ” 
generally mean their agnatic relations, and none of my agnatic relations, 
except my brothers, have the defect. — Yours faithfully, B^. 
IQtJi June 1893. 
Deak, Sik, — In the thirteen years since I wrote to you, the descendants of 
my colour-blind relations have largely increased. But I know of no case of 
colour-blindness among my nephews, nieces, first cousins once removed, second 
cousins once removed, or third cousins once removed. B^^ daughter is lately 
married. It will be curious to see if she has a colour-blind son. I believe 
that a daughter of J^g’s married some time ago, but I don’t know whether she 
has a family or not. I think these two are the only two directions in which 
colour-blind children can be immediately expected. 
It is rather noticeable that the (a) line differs from the (b) line, in that the 
eldest son of was not colour-blind, whereas the eldest son of b was partly 
colour-blind, and the eldest sons of bo, bg, h^, bg, bg are all colour-blind. This 
seems to suggest that the eldest son of the colour-blind man’s daughter is 
peculiarly liable to inherit the defect. 
My brothers and I had noticed the law of transmission several years 
before 1880, but I neither remember the exact date nor to which of us it first 
occurred to do so. We had never read anything on the subject. — Yours 
faithfully, B^. 
The above description being necessarily somewhat complicated, I have tried 
to express the course of descent of the malady, through the five generations, 
more clearly in the following diagram, where the italics distinguish the colour- 
blind descendants. W. P. 
Genera- 
tions. 
I. 
Ancestor, A 
(Assumed to be Colour-blind). 
II. 
Daughter, 
a. 
Daughter, 
b. 
Daughter 
c. 
III. 
Son, 
Ag. 
Son, 
B. 
S 07 l . 
Son. 
IV. 
Daughter, 
U2. 
Daughter, 
bg. 
Daughter, 
bg. 
Daughter, 
b4. 
Daughter, 
bs. 
Daughter, 
be. 
— 
V. 
(Present.) 
Son, 
^3- 
Son, 
B-2. 
Son 
^3- 
Son, 
Son, B^, 
S 07 l , Bq. 
Son, B’j. 
S 07 %, 
Bs- 
