Some Characters in Human Beings 43 



In most families it appears to be the result of a dominant gene 

 but in some instances is due to a recessive. In nystagmus the 

 eyeball shows a continuously rolling movement. In some families 

 this is the result of a recessive gene. In others it is caused by a 

 dominant and the character, itself, is somewhat different for 

 the rolling of the eyeball is accompanied by movements of the 

 head. Nystagmus may be caused by environmental conditions 

 as well as by genes. Some injuries to the brain and some brain 

 tumors may cause the same condition as that brought about by 

 the dominant gene. This should caution us against a too hasty 

 judgment as to the inherited or noninherited nature of a certain 

 character. 



An interesting ear abnormality reported by Potter is the result 

 of a dominant gene which causes the pinna to be small, deformed, 

 and inverted. Several genes that affect hearing are on record. 

 Deaf-mutes are born totally deaf. This character may be the 

 result of either one of two recessive genes, d and e, or both. If 

 both dominant alleles are present, the individual is normal, but 

 if a person is homozygous for either d, or e, or both, he is a deaf- 

 mute. Other inherited conditions of deafness are labyrinthine 

 deafness, in w^iich the auditory nerve begins to degenerate at 

 about forty years of age, and otosclerosis, a progressive deafness 

 which begins at about thirty. 



A number of inherited tooth defects have been observed. 

 Dominant genes have been identified which cause such abnormali- 

 ties as absence of the upper incisors, absence of certain incisors 

 and molars, lack of permanent upper canines, lack of two or more 

 wisdom teeth, supernumerary teeth, and defective enamel, result- 

 ing in brown teeth. The two center incisors of both jaws are 

 missing in some families as the result of a recessive gene. 



In addition to various structural traits, we find that physio- 

 logical processes and susceptibilities to various diseases may 

 come under gene control. The ability to taste certain substances 

 is an interesting character. Blakeslee and some of his co- 

 workers showed that about seven people out of ten can taste 

 crystals of phenylthiocarbamide. To them, it generally tastes 

 very bitter, although the strength of the taste varies with dif- 

 ferent people, and to some it appears to be salty. To the other 

 30 per cent it is tasteless. An examination of a number of 

 families has shown that the ability to taste is due to a certain 



