Chap. 20 THE PHYSICAL BASIS OF HEREDITY 415 



female the influence of an earlier mating may be carried on to the offspring 

 that result from the later one. This is the favorite reason that dog breeders 

 propose when puppies have been due to "mistakes." Similarly, among cattle 

 dealers there is a notion that if a "blooded" bull is mated to a "scrub" cow, 

 the latter may infect his offspring of later matings. Such beliefs are numerous 

 but have no foundation in fact. 



Human Inheritance 



Value of Knowledge. Knowledge of human heredity is of great practical 

 value, (1) in medical treatment, especially public health, (2) in forming wise 

 opinions and judgment of the special and economic problems that crowd the 

 present world, and (3) as an aid in reaching legal decisions, such as disputed 

 parentage. With a knowledge of heredity it is also to be remembered that chil- 

 dren cannot choose their parents. The parents do the choosing; the children 

 take the results. 



The inheritance of many physical and mental defects and diseases is becom- 

 ing more or less clearly understood. It is important to know whether a defect 

 is a dominant or a recessive since no recessive even if present in one parent 

 will crop out in a child unless a matching recessive is transmitted by the other 

 parent. At present, prospective parents can secure a clearer idea than ever 

 before of what benefits or dangers they may pass on to their children. There 

 are blood tests which detect the presence of hemophilia and hereditary anemia 

 in carriers who otherwise give no hint of the diseases. 



Genetics holds a leading role in the investigations of cancer and thousands 

 of experimental studies are being made in this field. Clues to the behavior of 

 breast cancer have been discovered in inbred mice and rats susceptible to the 

 disease. Globular particles (the "milk factor") visible under the great mag- 

 nification of the electron microscope have been isolated from these inbred mice 

 that regularly transmit cancer to nursing offspring. 



Heredity is in the kernel of racial problems. There is at least a better chance 

 for clearer thinking and wiser judgment about social problems when the facts 

 of human inheritance are kept in sight. The facts overtop the notions of pure 

 human breeds and superior races. All human beings are multiple cross breeds. 

 All are superlative mongrels, that are like kaleidoscopes whose patterns may 

 be changed but only insofar as the material allows. 



Knowledge of blood types has entered the courts, as in New York, where a 

 man claimed that he was not the father of his wife's child. The tests showed 

 that his blood was type O, "universal donor"; his wife's type was A, and the 

 child was AB. Since the parents could not pass on a combination of genes for 

 the AB type, the court decided with the father. 



Examples of Inherited Qualities. Blood is an extremely sensitive and complex 

 chemical compound. Even in closely related species of animals the chemical 



