132 



Dihybrids, Trihyhrids, and Polyhybrids 



1 SS WW — deep spots, brown lint 



2 SS Ww — deep spots, cream lint 

 2 Ss WW — pale spots, brown lint 

 4 Ss Ww — pale spots, cream lint 

 1 SS WW — deep spots, white lint 



2 Ss WW — pale spots, white lint 



1 ss WW — no spots, brown lint 



2 ss Ww — no spots, cream lint 

 1 ss WW — no spots, white Unt 



16 



If there is complete dominance in one pair of genes but incom- 

 plete dominance in another pair, the r2 ratio is still different. 

 Von Tschermak showed that in barley normal spike is completely 

 dominant to branched and that tall culm is incompletely domi- 

 nant to short. If a homozygous normal-spiked, tall plant 

 (BB SS) is crossed with one homozygous for branched-spike 

 and short culm {bb ss) theTi is normal-spiked with culms of 

 intermediate height. The F2 should then segregate into: 



1 BBSS 



2Bh SS 



2 BBSs 



4B6 *Ss 



IBBss 

 2Bb ss 



3 normal, tall 



1 bb SS branched, tall 



2 bb Ss branched, intermediate 

 \ 6 normal, intermediate 1 bb ss branched, short 



[ 3 normal, short 



Testcross. In maize the Fi between a homozygous purple, 

 normal-leaved plant and a green, crinkly-leaved plant is pheno- 

 typically purple and normal-leaved and produces four kinds of 



"Checkerboard" Method 



Gametes of Recessive 

 pier 



Testcross 



Method of Combined Ratios 



Testcross 

 Ratio of Plpl 



Testcross 

 Ratio of Crcr 



I PlCr 

 ^Plcr 

 ^ plCr 

 fa plcr 



Plpl Crcr 



Plpl crcr 



plpl Crcr 



plpl crcr 



Combined 

 Testcross Ratio 



lCrcr——l PlplCrcr 



1 crcr 1 Plpl crcr 



1 Crcr 1 plpl Crcr 



1 crcr 1 plpl crcr 



Fig. 44. The result of testcrossing or backcrossing an Fi plant heterozy- 

 gous for PI and Cr with the double recessive. Left, the checkerboard 

 method, and, right, the method of combined ratios. 



gametes in equal numbers: PI Cr, PI cr, pi Cr, and pi cr. If this 

 Fi is backcrossed to a crinkly-leaved, green plant, the ratio of 

 offspring will be 1 : 1 : 1 : 1, shown by a checkerboard (Fig. 44) 

 or by combining the testcross ratios of the individual pairs of 

 alleles. The testcross can be used for two independent genes in 



