Two Pairs of Genes in Two Pairs of Autosomes 127 



(telomitic) and one in which the centromere is not at the end 

 (atelomitic). That these can be arranged at metaphase in four 

 different ways so as to give eight different gametes (Fig. 40) 

 shows that the members of one pair segregate entirely inde- 

 pendently of other pairs. 



UI fti f»J H« 



Fig. 40. Independent segregation of chromosomes. Three pairs of het- 

 eromorphic chromosomes are present and separate in all possible combina- 

 tions. (Redrawn from Carothers in the Journal of Morphology.) 



Recombination. In a monohybrid the gametes of the male 

 unite in an entirely random fashion with the gametes of the 

 female. It is also true when the Fi is heterozygous for more 

 than one pair of genes. Let us illustrate this point in a dihyhrid, 

 where the Fi is heterozygous for two pairs of genes. In the 

 cross between PlPl CrCr and plpl crcr the Fi is Plpl Crcr. Four 

 kinds of eggs and four kinds of male gametes are formed by the 

 Fi plant. They are genetically PI Cr, PI cr, pi Cr, and pi cr. 

 Any male gamete can unite with any egg, and in fact does so 

 with equal frequency. The result of this combination of four 

 different eggs with four different male gametes would be sixteen 

 plants, and those sixteen possibilities can be illustrated by a 

 diagram known as a "checkerboard" (Fig. 41). When the 

 various kinds of genotypes are tabulated the result is: 



9 purple color, normal leaves 



Genotypes Phenotypes 



1 PlPl CrCr' 



2 PlPl Crcr 

 2 Plpl CrCr 

 4 Plpl Crcr 



2 Plpl axr I ^ P^^Pl® ^o^o^' crinkly leaves 



_ , , -^ is green color, normal leaves 

 2 plpl Crcr J 



1 plpl crcr 1 green color, crinkly leaves 

 16 16 



