686 



HEREDITY AND CELL DIVISION 



simplest way of finding their possible combinations in their correct 

 proportions is to draw a rectangle subdivided into squares equal 

 in number to the product of the numbers of kinds of gametes 

 produced by the two parents. Thus in Mendel's experiment 

 with round yellow and wrinkled green seeds the F^, which were 

 heterozygotic round yellows, must have been capable of giving 

 gametes which carried the factors 



RY 



Ry 



rY 



When we cross two of these plants we must therefore draw a 

 rectangle with 4 x 4 = 16 squares (Fig. 533). The gametes of 



Fig. 533.- 



-'Chequer-board' or Punnett square for a two-factor cross; round yellow, 

 and wrinkled green seeds. 



one parent are then written into all the horizontal rows, and of 

 the other into all the vertical rows. The factors which now appear 

 in each square represent a possible zygote. If like terms are 

 collected (i.e. factors for the same pair of allelomorphs) it will 

 be seen that the possible zygotes are in the proportions shown 

 in Table X, and when allowance is made for dominance, give 

 the 9:3:3:1 ratio which Mendel actually found. What we have 

 done by this ' chequer-board ' is simply a piece of mechanical 



