238 ESSENTIALS OF ZOOLOGY 



for a breeder to bring about any desired combinations of the traits 

 possessed by the species of plant or animal with Avhich 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 independent, 

 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 orig- 

 inal dwarf stock. To illustrate both the principles of unit characters 

 and the related phenomenon 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, at a distance at 

 least, would look 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. The white marbles and the black marbles can be segregated 

 out again if it is so desired and either variety can then be put into 

 combination with marbles of any other kind. Inheritable traits, like 

 marbles, can make various combinations which last for one generation 

 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 almost 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 con- 

 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 

 conditions, 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. 



