centuries; and such records were not being kept so long ago. The number of 

 offspring in human ma tings is comparatively small. We can therefore never 

 get even a hint of all the possible character-combinations in any one family. 

 Since human mating normally involves so many elements of taste, sentiment, 

 affection, and other feelings and values, experiments are out of the question 

 among free people. Finally, what we call the human race is really a mixture 

 of many distinct types or combinations of characters, and these are so thor- 

 oughly mixed up that we cannot find a "pure" race of human beings at the 

 present time. It has nevertheless been possible to compare the facts obtained 

 from family records with the behavior of various characteristics in the pedi- 

 grees of plants and animals. Such studies show that many human characters 

 reappear in families according to the hereditary principles of dominance, 

 segregation, recombination, and linkage. 



We shall probably apply our knowledge of heredity to human affairs along 

 the line of showing what types of marriage are likely to produce offspring with 

 one or another undesirable trait. We already know that certain abnormalities 

 of physical structure or mentality are transmitted in a definite way. We 

 therefore counsel men and women in whose families certain undesirable reces- 

 sive characteristics appear not to marry others of similar stock. In the course 

 of time we shall no doubt develop certain standards of fitness for marriage 

 which will be enforced largely by the same kind of public opinion and tradi- 

 tion as now distinguish the customs of different peoples. 



In Brief 



Of the many characteristics, or traits, present in any organism, certain ones 

 are transmitted, or inherited, independently of certain others. 



With regard to a pair of alternative traits, a hybrid may resemble one 

 parent completely, presenting the dominant character, and not show in its 

 appearance or behavior the possibility of transmitting the alternative recessive 

 quality. 



Individuals that are pure-recessive for a given character breed true, as do 

 individuals that are pure-dominant; but hybrids cannot transmit the dominant 

 character to all their offspring. Matings of hybrids result in segregation, or a 

 breaking up of the combinations of characters derived from different ancestors. 



To say that a plant or animal has inherited certain characters from the 

 parents means that there is something in the zygote, or fertilized tgg, which 

 makes possible the development of those traits, and that whatever is in the 

 zygote must have come from the gametes and so, presumably, from the parents. 



The recurrence and disappearance of certain peculiar traits in successive 

 generations agrees with the behavior of the chromosomes in plants and ani- 

 mals during the formation of gametes, during fertilization, and during develop- 



502 



