6 MENDELISM chap, i 



the nature of the problem underlying the process of 

 heredity. 



Owing to their peculiar power of growth and 

 the relatively large size to which zygotes attain, 

 many of their properties are appreciable by observa- 

 tion. The colour of an animal or of a flower, the 

 shape of a seed, or the pattern on the wings of a 

 moth, are all zygotic properties, and all capable of 

 direct estimation. It is otherwise with the properties 

 of gametes. While the difference between a black 

 and a white fowl is sufficiently obvious, no one by 

 inspection can tell the difference between the egg 

 that will hatch into a black and that which will hatch 

 into a white. Nor from a mass of pollen grains can 

 anyone to-day pick out those that will produce white 

 from those that will produce coloured flowers. 

 Nevertheless, we know that in spite of apparent 

 similarity there must exist fundamental differences 

 among the gametes, even among those that spring 

 from the same individual. At present our only way 

 of appreciating those differences is to observe the 

 properties of the zygotes which they form. And as 

 it takes two gametes to form a zygote, we are in the 

 position of attempting to decide the properties of 

 two unknowns from one known. Fortunately the 

 problem is not entirely one of simple mathematics. 

 It can be attacked by the experimental method, and 

 with what measure of success will appear in the 

 following pages. 



