268 Notes on the Pic/mentation of the Human Iris 
or (b) duplex and simplex offspring in the proportion of about 3:1; (3) duplex 
parents mated with simplex parents give either (a) all duplex offspring, or 
(b) duplex and simplex offspring in the proportion of about 1 : 1." 
He adds " It is evident therefore that the simplex type, in heredity, behaves 
as a Mendelian recessive to the duplex type, which is dominant." " The unit 
characters concerned in the heredity of the duplex and simplex types of eyes are 
therefore presence (duplex) and absence (simplex) of anterior pigment in the iris, 
presence being dominant over absence, which is recessive." 
From the above statements it follows : 
1. That in studying the pigmentation of the human iris, the two surfaces — 
anterior and posterior — alone are of importance ; the stroma which is about ten 
times as thick as the posterior layer of retinal pigment epithelium is negligible. 
2. That two kinds of pigments may be present in the human iris, the one 
yellow-brown in colour located on the anterior surface, the other blue-black in 
colour located on the posterior surface. 
3. That Mendelian laws regulate the inheritance of these types. 
Since the publication of the paper cited, I have, as occasion allowed, in the 
out-patient department of the Eye Institution in Aberdeen, where the annual 
attendance is over 20,000, taken notes of eye-colours and pedigrees which might 
throw any light on this question. My first difficulty was in finding any eye in 
which I could state, after careful examination in daylight and by focal illumination 
with magnification, that there was no anterior pigmentation. This difficulty still 
continues, and any such case noted in this paper must be taken to mean that no 
anterior pigmentation was seen at the examination. Several times I have thought 
that I had discovered a pure blue, or pure grey, or pure blue and grey parentage 
with no anterior pigmentation, but on careful examination traces of pigment were 
found in one or other parent. 
My next difficulty lay in the immense number of different types of pigmenta- 
tion. Only very few (each covering many grades) can be given in this paper, and 
these refer solely to the pedigrees of the families mentioned. This multiplicity 
of types probably accounts for the following differences from Hurst's observations 
as noted at pp. 86, 87 of his paper. 
1. " Self-coloured duplex " eyes are often " spotted." 
2. " Ringed duplex " eyes are often " spotted." 
3. The pigment ring of " ringed duplex " eyes is often outside the pupillary 
zone, occupying an irregular middle zone, and also sometimes, in spots, a peripheral 
zone of the iris. 
4. " Self-coloured duplex " eyes occur with no denser pigment ring round the 
pupil. 
5. Eyes with grades of anterior pigmentation so low that they can be 
mistaken for " simplices " do occur. 
