MENDEL'S LAWS OF INHERITANCE 



287 



result. We have already seen that in peas, yellow is dominant to green, 

 and round is dominant to wrinkled, so that the nine classes of genotypes 

 will produce four kinds of phenotypes as follows: 



Genotype GG WW 1 in 16 gives phenotype of yellow and round 

 Genotype GG Ww 2 in 16 gives phenotype of yellow and round 

 Genotype Gg WW 2 in 16 gives phenotype of yellow and round 

 Genotype Gg Ww 4 in 16 gives phenotype of yellow and round 



Genotype GG ww 1 in 16 gives phenotype of yellow and wrinkled) 

 Genotype Gg ww 2 in 16 gives phenotype of yellow and wrinkled/ 



Genotype gg WW 1 in 16 gives phenotype of green and round 1 

 Genotype gg Ww 2 in 16 gives phenotype of green and round / 



Genotype gg ww 1 in 16 gives phenotype of green and wrinkled . . 1 



If we compare this theoretical expectation with the numbers of each 

 phenotypic class of the 556 F 2 peas that were actually obtained, we find 

 that the correspondence is very close : 



The principle of independent assortment that Mendel assumed in 

 order to account for the four phenotypic classes of the dihybrid F 2 was 

 clearly substantiated by further breeding experiments and today is often 

 referred to as MendeVs second law. It may be stated as follows: When 

 more than one pair of allelomorphic genes are involved in a cross, each 

 pair assorts independently of the others. 1 



THE TRIHYBR1D CROSS 



Mendel's most elaborate experiment was the crossing of homozygous 

 parent stocks that differed in regard to three sets of characters, or three 

 pairs of genes. One stock was characterized by having round seeds, 

 yellow cotyledons, and a colored (gray-brown) seed coat; the other by 

 having wrinkled seeds, green cotyledons, and a colorless (white) seed 

 coat. This cross is given in diagrammatic form on the next page. It 

 will be seen that the new trihybrid phenotypic F 2 ratio 27:9:9:9:3:3:3:1 

 can be accounted for by the principles already encountered — segregation 



1 Later we shall see that under certain conditions Mendel's second law does not 

 hold, and that it needs additional qualifications. 



