44 Genes and Characters 



dominant gene and that the homozygous recessives detect no 

 taste. Other genes are known that affect the sense of smell and 

 determine whether an individual can detect certain odors. 



Other characters that have been reported are hereditary epis- 

 taxis or bleeding of the nose. It is due to a dominant gene and 

 may be associated with red spots on the skin and a general sus- 

 ceptibility to colds and nasal infections. An inherited tendency 

 for susceptibility to certain diseases has been noted in many 

 families. A recessive gene for susceptibility to poliomyelitis has 

 been discussed. Recessive genes have also apparently been dis- 

 covered that produce susceptibility to tuberculosis, scarlet fever, 

 and diphtheria. Studies of inheritance of diseases of this nature 

 are complicated by the fact that a causative infectious agent 

 must be present for the disease to be contracted while other 

 conditions such as general health, insufficient food, and certain 

 conditions under which a person works may contribute to his 

 susceptibility. Considering, however, that every person who 

 is exposed to the disease does not contract it, and considering 

 the pedigrees of a number of people who have contracted it, 

 susceptibility or resistance to these diseases appears to depend 

 in part upon the homozygous condition of a certain recessive 

 gene. 



Sugar diabetes or diabetes mellitus is reported to be inherited 

 as a recessive. It may also, however, be caused by syphilis, 

 other diseases, or certain emotional states. That a disease may 

 sometimes be caused solely by environmental factors does not 

 preclude the possibility that it may sometimes be inherited. 

 Actually, this disease may be caused by anything that disturbs 

 the functioning of the islands of Langerhans in the pancreas, 

 resulting in a normal secretion of these structures in a lower 

 amount than in healthy individuals. The cause of the disturbance 

 may be a disease or an emotional condition or a gene. 



Certain genes appear to disturb the activity of the thyroid 

 gland, producing such conditions as goiter. Apparently a re- 

 cessive gene produces alkaptonuria, a condition in which a 

 certain acid, alkapton, is present in the urine, causing it to turn 

 dark. A very rare condition is steatorrhea, a condition in which 

 fat is not digested. It appears to result from one or more pairs 

 of recessives. Another very rare condition, reported by Macklin, 

 is porphyrinuria. Caused by a homozygous recessive, it results 



