MENDELISM 



125 



gametes d and two female gametes r, we arrange them in a 

 square in such wise that the similar gametes are in vertical 

 lines. We then make another square, which similarly 

 contains the male gametes, but so arranged that the 

 similar gametes now lie horizontally next to each other. 

 If we now superimpose the two squares, we get all the three 

 possible combinations of male and female gametes in their 

 numerical proportion. 



FIG. 50. PUNNETT'S SQUARE FOR HYBRIDS. 



A, female gametes arranged vertically ; B, male gametes arranged 

 horizontally ; C, male and female gametes superimposed. 



Thus we get, on inbreeding of the first hybrid, offsprings 

 of the following compositions : I dd, I dr, I dr, i rr, or 

 i dd+2 dr +i rr. (As d is dominant over r, the 2 dr 

 will in appearance be like d.) We see from this that the 

 extracted pure dominant of the F 2 generation is a homo- 

 zygote of the character dd, the extracted recessive a homo- 

 zygote of the character rr, while the impure dominant is a 

 heterozygote containing the characters dr. (This is the 

 reason for writing in the diagram Fig. 48 the extracted 



