824 TEXTBOOK OF ZOOLOGY 



possessed by the species of plant or animal with which he is working. 

 Thus the modern Shasta daisy has a combination of the three impor- 

 tant characters: size of flower, gloss of petal, and prolific growth, 

 each of which was procured from an original variety having one, but 

 not the other two, of these traits. 



Principle of Segregation. — Since unit characters are independ- 

 ent, it follows that they can go into various combinations and 

 are free to segregate out again. This was illustrated by dwarf- 

 ness in Mendel's peas which went into a cross with tallness, 

 but segregated out again to form peas that were as consistent 

 for dwarfness as was the original dwarf stock. To illustrate 

 both the principles of unit characters and the related phenome- 

 non of segregation we might use an analogy. If a gallon of white 

 marbles were mixed with a gallon of black marbles, they would make 

 a mass of marbles that would look speckled-gray. But the mixture 

 is not irrevocably gray as would be the case if a gallon each of black 

 and white paint had been mixed. Inheritable traits, like marbles, 

 can be placed in various combinations. These may last for one genera- 

 tion only, after which they can make new combinations with other 

 traits. 



The Physical Basis 



With the simple facts in mind of how heredity works, it is desirable 

 to consider the germinal background of the laws governing it. 



The tiny sperm cells of both plants and animals and the egg cells 

 which, deprived of their food-containing yolk, are about equally 

 small, contain within them something that determines all the traits 

 which the individual resulting from their combination will develop. 

 In fact, the body of the sperm cell and the nucleus of the egg eon- 

 tain little else than a material that carries the combined inheritable 

 traits of one generation over to the next. This important material is 

 called chromatin and during mitosis it becomes arranged in series of 

 hereditary units called genes. Each gene has a definite causal rela- 

 tionship to an inheritable trait. Both the sperm cell and the egg cell 

 contain a complete gene complex, and each, under favorable condi- 

 tions, could produce a complete individual. 



When the two totipotential gametes are brought together at fertili- 

 zation, the resulting zygote contains two genes for each allelomorph. 

 When the two genes are for the same trait, the resulting individual 



