362 LAW OF INDEPENDENT ASSOCIATION 



square after the man who suggesteci its use) allows one 

 to determine readily what combinations are possible and 

 in what ratio each will occur. Along the top of the dia- 

 gram are written the four kinds of eggs and along the 

 left side the four kinds of sperms. In the sixteen squares 

 are written the various combinations which four kinds of 

 sperms can make with the four kinds of eggs. 



Fig. 102 is Punnett Square showing recombinations of the gametes 

 of the tall purple hy})rid. 



The 9:3:3: 1 Ratio. — It was stated in a previous para- 

 graph that there would be four combinations of characters 

 in the second hybrid generation when the tall purple hy- 

 brids are self-pollinated and that they would occur in the 

 ratio 9:3:3:1. These four types can be readily identified 

 in the diagram. Every one which has either one or two 

 " Tall " genes T will be tall, and likewise everyone with 

 one or two " Purple " genes P will be purple. Thus, Nos. 

 1, 2, 3, 4, 5, 7, 9, 10, 13 are all Tall Purple, and are nine 

 out of the total of sixteen. Similarly, Nos. 6, 8, 14 are 

 Tall white and are three out of sixteen. Nos. 11, 12, 15 

 are short Purple and three out of sixteen. No. 16 is short 

 white. This ratio is readily seen to be a general one 

 for any two independent dominant characters. 



Genotypes and Phenotypes. — This diagram also 

 makes clear another important fact. There is only one 



