THE PRINCIPLES OF HEREDITY 509 



ever, and this is the important point, if we tabulate the eye color of 

 thousands of children from many families with the particular gene 

 combination of this couple, we shall find that almost exactly one half of 

 them will have blue eyes and the other half will have brown eyes. The 

 law of averages works out, and the ratios can be expected to run very 

 true in such large numbers. 



This illustrates the advantage of using animals that bear large 

 numbers of offspring for genetic studies and again shows why human 

 beings are poor subjects for such studies. We can study a large number 

 of pedigrees of human families, however, and compare the results with 

 those obtained by breeding experiments with other animals and thus 

 learn much about human heredity. Probably the majority of the in- 

 formation we have on heredity has come from genetic crosses of the tiny 

 fruit fly, Drosophila, which we commonly see buzzing around garbage cans 

 during warm weather. A single pair of these flies in a small vial with a 

 few cents' worth of corn meal or banana food will yield a hundred or 

 more offspring within about ten days' time. These are sufficient to ob- 

 tain reliable ratios. Information obtained by such crosses has unlocked 

 many of the secrets of heredity in other forms of life including man. 



Intermediate Genes 



There are some characteristics which are influenced by genes which 

 are neither dominant nor recessive, but intermediate in their effect 

 when heterozygous. As an example, if we cross a pure white, short- 

 horned cow with a pure red, short-horned bull, the resulting calf will 

 be neither red nor white, but roan. Roan is a color which is an inter- 

 mediate expression of the red and white genes. Neither of the genes 

 is dominant over the other and both are partially expressed. When 

 roan cattle are crossed together, the offspring are red, roan, and white 

 in the ratio of 1:2:1. This shows that the genes do not mix and that 

 there is not a specific gene for roan. 



One of the most interesting cases of intermediate inheritance in 

 man is concerned with the blood types. When a person with type A 

 blood, who is homozygous, marries a person with type B blood, who 

 is homozygous, all of their children will have type AB blood. Neither 

 the gene for type A nor the gene for type B is dominant, so both blood 

 antigens are formed. When two persons with type AB blood marry 

 they will have children with type A, type AB, and type B blood in the 

 1:2:1 ratio. The gene for type O blood, on the other hand, is recessive 

 to both the gene for type A and the gene for type B. Hence, if a 

 homozygous type A person marries a type O person, all of the children 



