THE POPULAR SCIENCE MONTHLY 



characters are involved. If one of the original parents had round seeds 

 and the other wrinkled seeds, these characters separate independently 

 of the green-yellow separation. "We say technically that the pair yellow 

 and green, and the pair round and wrinkled segregate independently 

 of each other. Chance combinations of the germ-cells in a double 

 hybrid of this sort give nine yellow round peas, three yellow wrinkled 

 peas, three green round peas and one green wrinkled pea (Fig. 2). 



The same rule applies to three or more pairs of characters. Mendel 

 assumed in fact that independent assortment 2 always takes place no 

 matter how many characters are involved. 



In more recent times evidence has been accumulating which shows 

 that the chromosomes are the bearers of most of the elements (factors) 

 that produce those characters that we study in heredity. I can not take 

 up the work that seems to me to place this hypothesis on a very prob- 

 able basis, but I shall simply assume that it is a reasonable conclusion 

 from the evidence at hand. 



Eggs 



Sperm 



F2 



F3 



t 



Fig. 1. Diagram to illustrate Mendel's law of segregation. Individuals (zygotes) 

 are represented by superimposed circles, whose colors stand for the factors involved. 

 Gametes (germ-cells) are represented by single circles. 



2 The term segregation applies strictly to the process of separation of the 

 contrasted factors (allelomorphs). When more than one pair is involved, the 

 distribution that follows the segregation of each pair is called assortment in 

 the text, and assortment is a different process from segregation; although it is 

 the resultant of segregation so far as each pair is concerned. 



