GENETICS 



229 



for they are in different pairs of chromosomes. As a result of this 

 independent distribution, four kinds of germ cells are produced, RW, 

 Riv, rW, and rw. There are these four kinds of eggs, about equally 

 numerous, and the same four kinds of spermatozoa, equally numerous. 

 In fertilization, random unions take place between each kind of egg and 

 each kind of spermatozoon — 16 combinations all told. 



Rough Black x Smooth White 



RRWW rrww 



RW rw 



Eggs' 



F; RrWw fRough Black) 



RW RW 

 Rw Rw 



rW rW 



rw rw 



Sperm 



Fig. 198. — Inheritance of two pairs of independent characters in guinea pigs, black 

 and white color, rough and smooth coat. The points on the backs of the animals indicate 

 rough coat. 



To write these 16 combinations in the F2 generation without omission 

 or duplication, it is convenient to use the Punnett square, so named 

 from the English geneticist who devised it. Such a square is included 

 in Fig. 198, which explains this cross. Each egg formula is written four 

 times, in one of the columns of four spaces down the chart. They are 

 put there to be fertilized by the four different kinds of spermatozoa. 



