42 Genes and Characters 



The color of the eyes is greatly affected by certain genes. In 

 albinos, previously mentioned, the eye lacks color entirely (ex- 

 cept for the pink color produced by blood in the blood vessels of 

 the eye) because of a homozygous recessive gene. In the pres- 

 ence of the allele of this gene, the eye is colored, but the specific 

 color depends upon the presence of other genes, the exact number 

 of which is not always easy to determine. One pair of alleles 

 seems to produce a basic brown or blue, but these, and especially 

 the brown type, are considerably affected by other genes. The 

 dominant of this pair, B, produces a purple-black color in the 

 uvea and choroid behind the iris and a brown layer in front of 

 the iris. Because of the latter pigment, the eyes appear brown. 

 In the homozygous recessives, only the first of these two layers 

 is present, and the eyes consequently are not brown and appear 

 blue or gray, depending upon the angle of reflected light, age of 

 the individual, and perhaps modifying genes. The brown type 

 can vary from a very dark brown to a light yellow-brown accord- 

 ing to the presence of various modifying genes. 



Eye defects are of many kinds. Dominant genes are known 

 which cause ectopis lentis or a congenital displacement of the 

 lens, aniridia, the complete lack of an iris, and glaucoma, a 

 defect in which the normal drainage of the lymph from the eye 

 is blocked and the retina becomes atrophied. Congenital cataract 

 is caused by a dominant gene with incomplete penetrance, for it 

 occasionally fails to appear in individuals possessing the gene. 

 Eye defects produced by recessive genes include a condition in 

 which the optic nerve becomes inflamed and atrophied, and 

 microphthahnus, in which the eyeball is very small and con- 

 sequently vision is impaired or the affected individual is blind. 



In some families a particular defect may be produced by a 

 dominant gene and in others by a recessive. Apparently during 

 the course of evolution, different genes appeared in different 

 individuals and probably at different times, producing the same 

 character or characters which are so nearly alike that they are 

 not separated into different categories. Examples include ex- 

 treme shortsightedness or high myopia, which is brought about 

 by a recessive gene that produces a globe of unusual length or 

 by a dominant that causes the cornea to be too greatly curved. 

 Farsightedness or hyperopia is a condition in which the globe is 

 so short that the rays are focused behind instead of on the retina. 



