522 Animal Biology 



berries, clams, eggs, sauerkraut, or certain drugs, (3) those due to con- 

 tacting the skin with wool, silk, feathers, etc. About 10 per cent of our 

 population suffers from some type of allergy which may be characterized 

 by such symptoms as gastrointestinal disturbances, respiratory disturb- 

 ances, skin eruptions, and even migraine headaches. Nonprotein allergies 

 may be Illustrated by formaldehyde, Iodine, aspirin, and certain sulfon- 

 amide drugs. 



Immunity in general may be considered as (1) natural or (2) acquired. 

 Natural immunity Is an Innate characteristic w^hlch Is determined genet- 

 ically and does not depend upon the reactions of the body when in con- 

 tact with an Infectious organism. It may be influenced by malnutrition, 

 fatigue, decreased temperature, certain anatomic structures, and certain 

 physiologic reactions and varies with the individual. Man and cattle 

 are naturally Immune to hog cholera. Acquired immunity is not Inherent 

 in the protoplasm of that species but must be acquired either actively or 

 passively. Immunity may be acquired actively by (1) having the dis- 

 ease, (2) being exposed repeatedly to quantities of the germ which are 

 not sufficient to produce the disease, (3) being treated by the products of 

 organisms. Immunity may be acquired passively by the administration 

 of immune sera which contain the proper antibodies that the patient has 

 had no part in actively producing. An advantage of passive immuniza- 

 tion Is the speed of protection, while its disadvantage is that it lasts only 

 a short time. 



XII. INHERITANCE OF HUMAN TRAITS 



Much Information regarding the inheritance of certain human traits 

 has been secured in recent years by a study of lower animals as well as 

 by the scientific study of human families. From these studies it is ap- 

 parent that many human traits are inherited according to Mendel's laws 

 just as are many of the traits of other animals and plants. Heredity of 

 plants and animals. Including man, is considered In Chapter 34. It will 

 be noted that some traits are not Inherited as simple Mendellan traits 

 but are due to sex-linked, blended, or multiple-gene Inheritances. One 

 of the most valuable methods of studvlns^ human inheritance is the scien- 

 tific assembling of accurate data In the form of "family trees" (Figs. 

 352 to 356). The correct interpretation of these data has contributed 

 much to our knowledge of the methods of inheritance of specific human 

 traits. Much is still unknown about certain traits, but additional Infor- 

 mation is constantly being added. Some of the difficulties encountered 



