590 



SCIENCE 



[N. S. Vol. XXVI. No. 670 



principals and other friends. The records 

 were made on blanks calling for the eye-color 

 through three generations. The total number 

 of cards — each giving the ancestry of one indi- 

 vidual — is 132, of which 57 are single cards 

 to a family while the remaining 75 are dis- 

 tributed in 20 families, an average of 3| chil- 

 dren to a family. 



Human eye-color falls into the main classes, 

 blue and brown. The blue color of the iris is 

 what is known as a structural color; no blue 

 pigment is present, but there is a small 

 quantity of scattered granules, reilection of the 

 light from which gives a blue color exactly as 

 reflection from suspended particles makes the 

 air blue. The black pigment of the choroid 

 coat gives a background that favors the reflec- 

 tion of light and prevents transmission ; in al- 

 binos, who have no black choroid coat at the 

 retina, light is reflected from the back of the 

 eye and the iris appears reddish by transmitted 

 -light even as the sky is red at sunset. Brown 

 eyes, on the contrary, contain melanic pigTaent, 

 reflection from which yields black. Thus the 

 blue eye is the absence of pigment. In addi- 

 tion to the two fundamental types we have 

 black eyes, due to a greater quantity of pig- 

 ment, and light (i. e., dilute) brown eyes. In 

 addition to black pigment the iris frequently 

 contains more or less yellow in specks or 

 patches. This is doubtless a fat-pigment or 

 lipochrome. The combination of black and 

 yellow pigment gives a green color as it does 

 in the green canary, and such green and blue 

 eyes are commonly called " gray." But 

 " gray " is also used for blue eyes with some 

 brown pigment in larger or smaller patches. 



The nomenclature of eye-color which col- 

 laborators were requested to employ was as 

 follows: Light blue, dark blue, blue-green or 

 gray, hazel or dark gray, light brown, brown, 

 dark brown, very dark brown or black. This 

 nomenclature was generally followed and 

 seemed to be understood except in the case of 

 " hazel," which we suspect was employed in 

 certain dark bluish-grays. The classification 

 was probably too detailed and the three groups 

 of blue, gray and brown would doubtless have 

 sufficed. In the following summaries minor 



divisions of these three fundamental groups 

 will frequently be neglected. 



The first result which an analysis of the 

 pedigree data reveals is that blue eye-color is 

 recessive to brown. The first evidence of 

 recessiveness is the purity of the germ cells of 

 the recessive type, so that when two recessive 

 individuals are mated inter se they throw only 

 the recessive type. Of the offspring of two 

 blue parents 69 are blue and 6 blue-gray or 

 gray. Two additional cases of so-called 

 " hazel " eyes we suspect to be of a blue type. 

 Again, whenever in one family, both father 

 and mother have blue eyes, all children have 

 blue eyes. This is true in the Ge. and Sw. 

 families of three children each, the Hur. 

 family of 4 children and the Ee. family of 

 6 children. 



The second criterion of recessiveness is the 

 absence of offspring of the recessive type from 

 parents one of which is of the recessive type 

 and the other a homozygous dominant. The 

 only family that seems to meet the conditions 

 of having a homozygous dominant brown 

 parent is a small one (Sa,) as follows: 



Children 

 Boy, dark brown 



Girl, dark brown 



light brown 



/ blue 



GrandpMents 

 f brown 

 1 brown 

 blue 

 ght blue 



rbl 



jli, 



A third criterion is found in crosses of the 

 E X DE type where a recessive is mated with 

 a heterozygous dominant; in this case there 

 should be an equal number of offspring of each 

 type. Six matings of this sort give 16 dark- 

 eyed to 9 light-eyed offspring — a deficiency of 

 the light-eyed group which is probably due to 

 the small numbers. 



Since blue or absence of pigment is reces- 

 sive we should expect to find some cases of 

 two homogametous dominant browns which 

 produce only brown-eyed offspring. We ap- 

 parently have one such family (McB.) in 

 which the four grandparents, two parents and 

 five children have all dark brown eyes. The 

 behavior of brown alone thus confirms that 

 of browns when crossed with blues, and all 

 results prove that black iris pigment is domi- 

 nant over its absence. 



