192 EVOLUTION AND GENETICS 



these genetic browns, for, so far as recorded they 

 have had at least one brown-eyed parent. In other 

 words, in extreme and exceptional cases possibly 

 due to weakness or disease the brown eye-color may 

 not develop in an individual that is genetically a 

 brown hybrid. This failure of the somatic character 

 does not affect the brown-producing gene, for, such 

 individuals behave in inheritance as hybrid brown- 

 eyed individuals. 



Albinism in man has been known for a long time 

 and the earlier records of white Indians seem, in the 

 light of recent discoveries, not to be mythical. In 

 all races, including negros, albinos are known. It 

 has been estimated that this occurs once in 5,000 to 

 once in 30,000 individuals. It is possible that there is 

 more than a single kind of albino due to mutation in 

 different genes or to allelomorphic mutations that 

 give different degrees of deficiency of pigment. 



Albinism is a recessive character. A few cases are 

 on record where two albinos have had only albino 

 children. In true albinos the brown pigment is ab- 

 sent from the skin, hair, and iris. Its absence in 

 the iris gives the eye a pink color due to the blood 

 in the back of the eye, showing through the semi- 

 transparent iris. 



A defect in vision known as stationary night- 

 blindness has been shown by Nettleship to be a 

 dominant Mendelian trait {fig. 73) . In one locality 



