Previous Chapter : V. HEREDITY OF COLOUR (continued) : PAGE 88.
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CHAPTER VI
HEREDITY OF COLOUR - CONTINUED.
Eye-Colours. Variations in Colour of the Iris – Deficiency of Eye-Pigments in some Coloured Types.
THE colours of eyes present some special genetic problems which are worth examining in detail.
Pigments are formed in several parts of the vertebrate eye, but chiefly in the choroid and the iris. In all
normal eyes the choroid contains much deep black pigment, and to this fact the blackness of the pupil is
of course due. The common differences in eye-colour with which we are familiar in man are not caused by
changes in the colour of the pupil but by variations in the structure and pigmentation of the iris alone.
Such differences, being conspicuous factors in individual and racial differentiation, have long attracted
the attention of anthropologists. With the object of elucidating the heredity of eye-colours extensive
pedigrees were collected by Galton, and biometrical studies of a similar kind have been published by
Professor Pearson and his assistants. Various conclusions of a statistical nature have been based on these
data, but as no critical analysis was attempted, such results are devoid of genetic importance. As we now
know, moreover, the method of classification employed in collection was unfortunately defective. The
categories were not sufficiently precise, and the material would give no reliable indications if analysis
were attempted.
As the result of comprehensive investigation carried out in a single district on Mendelian lines, Hurst
has succeeded in making an important advance. Recognizing that such descriptions as "light," "dark," and
so forth are
* Similar evidence has also been published by Davenport (107).
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too vague he instituted a more critical classification, which led to the discovery that the presence of brown
pigment on the anterior surface of the iris behaves as a dominant to the absence of such pigment, which is
a recessive character.
Parents without the pigment in question have exclusively children who are similarly without it,
whereas persons possessing the brown pigment may be either homozygous or heterozygous in that
respect, and their offspring follow the numerical rules with fair exactitude.
Fig. 18. Two pedigrees showing the descent of eye-colour (after Hurst).
[Figure not reproduced in this version.]
"Brown" eyes are no doubt all dominants, and "blue" eyes are very often recessives ; but on the other
hand, many eyes , which would be called "light" have some of the
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pigment, and many eyes which would be called "dark" are without it. The differences between the nonpigmented "dark" iris and the non-pigmented "light" iris are caused by structural differences in the iris
itself.
Apart from its scientific importance it will be evident that this discovery is of considerable practical
interest as a contribution to human genetics. Much nevertheless remains to be done in the application of
precise methods to the problem of human eye-colours. For example, amongst the dominants examined by
him Hurst recognized two distinct types : (1) self-coloured irides in which the pigment is distributed over
the whole surface of the iris, (2) ringed irides in which the pigment was absent from the periphery but
existed as a ring round the pupil. The relations between these two types could not be satisfactorily
determined. All that the evidence showed was that the "self" behaves as a dominant to the "ringed." But
on analogy with other such cases we of course should expect that these distinctions were caused by
distinct determining factors, either of which might be borne by the non-pigmented recessive without
revealing its presence. If this were so, some of the non-pigmented should on breeding with the ringed,
have off-spring with self-coloured irides, just as some albino rabbits on breeding with Dutch-marked
rabbits may have self-coloured offspring. Curiously enough, however, in an ample series of observations,
Hurst met with no such occurrence, though examples of self parents producing the three kinds of
offspring, self, ringed, and non-pigmented, were not rare. Here, at present, the matter rests*.
Hurst's observations relate entirely to the population of a small English village where naturally only a
limited range of types would be met with. The inter-relations of the eye-colours of strongly marked racial
types doubtless will provide an excellent field for the application of a similar analysis. It is likely too that
such work would result in the discovery of the principles governing the association of certain eye-colours
with particular colours of the hair, and
* Hurst has suggested with some plausibility that the absence of any self-coloured irides among the offspring of ringed x
non-pigmented may be, attributed to the fact that the offspring examined were mostly school children. He thinks it not
improbable that some of the ringed may subsequently develop into self-coloured irides.
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other characteristics. In all such inquiries the first step is to distinguish the critical differences which are
treated as units in the constitution of the germ-cells.
In addition to these observations on man we have a few indications as to the heredity of iris-colours in
birds.
The iris of the domestic breeds of fowls presents at least three types of coloration. The common colour
in the full grown birds is a bright red, apparently due to the formation of a red pigment when the period of
adult life is approached, the colour in earlier stages being a dull blackish green.
Malay fowls are peculiar in having a pale, yellowish white iris -the "daw-eye" of fanciers- which
behaves as a recessive to the red iris. This eye may be found in birds of various colours and probably it
always indicates an admixture of Malay blood. I have seen, for instance, a daw-eye in a White Leghorn of
a good strain. Here it undoubtedly cropped out as a recessive character among the normal red-eyed birds,
and I happen to know that White Malays were used by a prominent White Leghorn breeder to increase the
size of his strain.
Another peculiar type is seen in the nearly black iris of Andalusians and Silkies. In Andalusians we
have found this eye imperfectly dominant to the red. The case of the Silkies is much more complex and
the inheritance is disturbed by sex in the manner hereafter to be described.
In the Little Owl (Athene noctua), Giglioli has described the occurrence of a remarkable variation to
eye-colour which is evidently an example of the appearance of a recessive in the wild state. The irides of
the normal birds are yellow, but in the variety, of which several specimens were found in one district, the
irides were black. The circumstances of the discovery which are related by Giglioli in detail† leave little
doubt that the condition was recessive. It is a curious fact that, in all the aberrant individuals found, the
distribution of the brown markings on the feathers also differed from that of the type. So distinct was the
variety from the ordinary A. noctua that a new specific name was suggested for it until the fact that it
could be bred from the type was established.
* Sir J. Bowring (Philippine Islands, 1859, p. 151) says that "white eyes" are preferred in fighting cocks. Probably these are
the origin of our Malays and Aseels.
† Giglioli, Ibis, 1903, p. 1.
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Deficiency of Eye-Pigment in some Coloured Types.
In the eyes of actual albinos, destitute of pigment, the eye has the well-known pink appearance. This is
due to the admission of light through the partially transparent iris. Generally speaking such great
deficiency of ocular pigments occurs only in total albinos, but to this rule there are a few remarkable and
interesting exceptions.
The best known example is the so-called Himalayan Rabbit*. In it the eyes look like those of the pure
albino though having a trace of pigment. At birth Himalayans are white or nearly so, but soon the muzzle,
ears, tail, and feet assume a deep chocolate-black. This type behaves as a recessive to the normal grey, and
to black, and it is a normal dominant to the pure albino. Hurst however found that Himalayans, though
obviously pigmented, may carry the factors for greyness and for self-colour, without any indication of
such factors being manifested in their own colour or in its distribution. Recent experiments made by
Punnett have given the same result.
The next example of the pink eye present in an animal not devoid of colour is seen in certain "Japanese
Waltzing" mice.
The "waltzing" is a peculiar vertiginous spinning to which those animals are subject. I understand that
the fact physiological nature of this movement has never been fully elucidated, but the condition has been
shown to be often, if not always, associated with malformation of the auditory labyrinth. As already
mentioned, the waltzing character behaves as a Mendelian recessive to the normal habit.
Such waltzing mice exist in various colours, some having ordinary black eyes, others pink eyes.
Various colours also may occur in the coats and we have not yet complete
* Darwin gives (Animals and Plants, ed. 21, 1885, 1 p. 113) facts as to the production of Himalayans by crossing Silvergreys. In the light of present knowledge it is no longer possible to accept these experiments as evidence of an original
production of the variety. Presumably the case was only one of the reappearance of a recessive form, and indeed Darwin adds
the remark that he had "recently been assured the pure Silver-greys of any such breed occasionally produce Himalayans."
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knowledge as to which of these colours are compatible with the pink eye and which are not.
Elaborate experiments were made by Darbishire (90) to test the inheritance of these mice. The waltzers
used were all pink-eyed and bred true to that character. In general terms the coat-colour was fawn-andwhite, but no detailed account of the pigments present has been published.
Crosses were made between these waltzers and ordinary albinos. The F1 generation so produced were
usually, as regards coat-colour, grey like wild mice, the shade being sometimes darker, sometimes lighter,
together with more or less white. In a few families the colour was black. The albino parents being of
miscellaneous origin, such diversity in F1 colour is what we should now expect.
In respect of eye-colour the remarkable fact was observed that the F1 mice were always black-eyed. In
F2 various coat-colours occurred, including a peculiar new type with exceedingly dilute pigment in the
hair, spoken of as "lilac." Among these coat-colours the eye-colours were distributed according to systems
not yet ascertained. All that can be clearly perceived is that the mice with agouti or full black in the coatcolour (whether pied with white or not) always had black eyes ; the completely white individuals always
had pink eyes ; and lastly, the eyes of the "lilac" mice were always pink*. The facts were unfortunately
presented by Darbishire in a form which makes further analysis impossible. A proper investigation of this
series of phenomena would have greatly increased our knowledge of the genetics of pigmentation.
The important conclusion may nevertheless be confidently drawn that at least two factors are
concerned in producing the black colour of the eye. Cuénot (88, p. II), as the result of his own
experiments, identifies these with (1) colour in the coat ; (2) a factor determining the blackness of the eye,
which needs the coat-colour factor as its complement. This is no doubt a part of the truth, but
* Miss .Durham (116) has shown that the pink eyes of these "lilac" mice and of the original pink-eyed waltzers are not
wholly devoid of pigment, but contain traces both in the iris and choroid visible in microscopic sections. The eyes of the albino
mice are absolutely without pigment.
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there are suggestions that it is not the whole, and that further complications have to be met. To discuss
these in detail is beyond the scope of this work, but it seems likely that the degree of saturation or dilution
of the coat-colour has to be considered.
There is evidently some intimate relation between the colour of the eye and the colour of the coat ; for
no mice have the coat-colour wild-grey ("agouti" of fanciers), black, or even blue, unless the eyes are
black. The interesting fact has also been discovered that the actual nature of the pigment of the choroid
differs in the different varieties*. Miss Durham finds that the chocolate mice always have chocolate eyes,
not black, no black pigment being present in either iris or choroid. The same fact has been published by
Castle (53) for the guinea-pig and was also noticed independently by Miss Sollas in guinea-pigs. From
these observations it is to be inferred that either the coat-colour controls the eye-colour, or the colour of
the eye controls that of the coat, but for various reasons it is not possible yet to declare positively which
account is the right one.
Guinea-pigs differ from other animals in that their albino strains generally have small "smudges" of
blackish pigment on some of the extremities, especially the tips of the ears, though the eyes are entirely
without pigment. It is said that albino guinea-pigs occur without these smudges, but I have not seen one.
The Cinnamon Canary must also be noticed here. This variety on hatching has pink eyes, though as
development proceeds, pigmentation supervenes. Many fanciers believe that they can recognize a
difference between the eyes of Cinnamons and those of other varieties, but the distinction is not readily
obvious. In plumage Cinnamons differ from other breeds in that the black pigment is absent and the
feathers consequently have a brownish tint. Miss Durham, from her observations, concludes that the
pigment which is developed in both eyes and feathers is chocolate, and that the Cinnamon is in fact a
chocolate
* John Hunter has an important paper on this subject : "On the Colour of the Pigmentum of the Eye in different Animals" (Obs.
on certain Parts of the Animal Economy, 1786, p. 199).
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variety of the Canary. The heredity of these characters has not yet been investigated by precise methods.
Fanciers however are agreed that some very remarkable phenomena of sex-limitation occur, which will be
described in connection with other instances of the same kind.
The only other example I know of a race which combines an albino eye with external colour is the
breed of cats called "Siamese." These animals, which breed perfectly true, were introduced from Siam,
where they have been kept for an indefinite period as pets of the royal household. Like the Himalayan
rabbit, Siamese cats are born almost white, but the fur becomes a curious fawn with darker chocolate
points on the ears and extremities. The eyes are more or less deficient in pigment throughout life. Some
pigment however is formed in the eye, as is evidenced by the fact that the iris has a blue colour and is
partially opaque.
From the little that is known respecting this group of cases it is clear that the genetics of eye-pigments
offer many features of interest. We have seen first that variations in the colour of the iris may exhibit
simple Mendelian heredity. Secondly when the general pigmentation of the eye is considered, it appears
that the presence or absence of such pigment may follow Mendelian rules ; but there is an interdependence between the factor or factors which produce the pigmentation of the eye and those which
govern the pigmentation of the fur or feathers, such that the eye-colour must be regarded in certain cases
(e.g. mice) as determined by two, or perhaps more, complementary factors. The actual nature of the
pigment in the eye is also variable, black being replaced by chocolate in several types destitute of black in
their hair. Lastly, there is a noticeable association of chocolate pigment, in hair or feathers, with the more
or less non-pigmented eye, as in the Himalayan rabbit, the Siamese cat, and the Cinnamon canary.
The peculiar phenomena connected with Albinism in Man are mentioned later in connection with
human heredity in general.
Next Chapter : VII. HEREDITY OF COLOUR (continued) : PAGE 115.
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