LAW OF INDEPENDENT ASSORTMENT 363 



plant out of sixteen which is white and short. It has both 

 recessive characters, and is pure (ttpp) for both. The 

 three short purples, however, are not all alike. No. 11 is 

 pure for both characters (ttPP) but both 12 and 15 are 

 pure only for short (tt) and hybrid for purple (Pp). The 

 same difference is to be noted among the tall whites and 

 tall purples; only one of each group is really pure, the 

 others being hybrid for one or both characters. From 

 this it appears that there are more combinations of genes 

 than are recognizable from the appearance of the plants. 



In order to distinguish the combinations or genes from 

 the combinations of characters biologists give them 

 separate names. Any particular combination of genes is 

 spoken of as a genotype, whereas any combination of char- 

 acters is called a phenotype, that is to say, visible type. 

 There are shown in the Punnett square 4 phenotypes or 

 visibly distinguishable types (i.e.. Tall Purple, Tall white, 

 short Purple, and short white), but there are nine geno- 

 types or combinations of genes, as follows: (1) TTPP 

 (No. 1), (2) TTPp (Nos. 2 and 5), (3) TtPP (Nos. 3 and 

 9), (4) TtPp (Nos. 4, 7, 10, 13), all of which look alike 

 and are tall and purple; (5) TTpp (No. 6), (6) Ttpp 

 Nos. 8, 14), all tall and white; (7) ttPP (No. 11), (8) 

 ttPp (Nos. 12, 15) all short purple; and (9) ttpp (No. 16) 

 short white. 



The Law of Independent Assortment of Genes 

 AND Characters. — When two parents which differ in 

 respect to two different character-pairs are crossed the 

 hybrid will exhibit both dominant characters irrespective 

 of which parent contributes them. 



When the hybrid forms its gametes the independent 

 genes, which are in separate chromosome pairs, segregate 

 and form four kinds of gametes. 



At fertilization the four kinds of gametes unite to form 

 nine different genotypes or combinations of genes, and 

 when both character-pairs show dominance four different 

 phenotypes or combinations of characters. 



