Polyhybrids 139 



"checkerboard," as illustrated ir; Fig. 47. When the various 

 types of the F2 are tabulated, the phenotypic and genotypic 

 ratios are: 



Phenotypic: 27ABC 9ABc MbC 9a5C 3.46c 3oSc 3abC lahc 



Genotypic: 1 AA BB CC lAABBcc lAAbbCC laaBBCC lAAbbcc laaBBcc laabbCC laabbcc 

 2 AA BB Cc 2 AA Bb cc 2AAbbCc 2 aa BB Cc 2 Aabb cc 2aa Bb cc 2 aa bb Cc 

 2AABbCC 2AaBBcc 2AabbCC 2aaBbCC 

 2 Aa BB CC 4 Aa Bb cc 4 Aa bbCc iaa Bb Cc 

 4 AA Bb Cc 

 4Aa BBCc 

 4Aa Bb CC 

 8Aa Bb Cc 



It is readily seen from the above that each kind of homozy- 

 gote is represented once; each genotype which is heterozygous 

 for one pair of genes, as AA BB Cc, AA Bb cc, aa bb Cc, is rep- 

 resented twice; each double heterozygote, as AA Bb Cc, Aa Bb cc, 

 is represented four times; and 8 individuals out of the 64 are 

 heterozygous for all three genes. This is easily understandable. 

 A plant which would be genotypically AA BB Cc, getting the C 

 gene from the motlier and the c gene from the father, would be 

 the same as one which received c on the female side and C on the 

 male, for once the two alleles get together in one plant or animal 

 any difference with respect to their parentage is lost. There- 

 fore, the two types, AA BB Cc and AA BB cC, would be classed 

 together. A double heterozygote would have two such pairs, so 

 there would be four individuals of each double heterozygote; the 

 triple heterozygote would have three pairs, and there would be 

 eight combinations of these which would be identical. 



Testcross. When an Fi plant or animal, heterozygous for 

 three pairs of genes, is crossed with a triple recessive, eight types 

 of offspring are produced in equal numbers, as shown in Fig. 

 47. The testcross from a trihybrid is, therefore, 1:1:1: 

 1:1:1:1:1. 



Polyhybrids 



Plants and animals that have more than three pairs of chro- 

 mosomes may naturally have more than three pairs of genes 

 which show independent assortment. Since in Drosophila me- 

 lanogaster there are four pairs of chromosomes, four pairs of 

 genes could show independent assortment, although one pair 

 would be located in the sex chromosome. In maize, ten pairs 



