160 THE MAINTENANCE OF SPECIES 



An illustrative case may serve to make both linkage and crossing- 

 over plainer. In corn, colored kernel (C) is dominant over colorless 

 kernel (c), and plump starchy grains (S) are dominant over wrinkled 



B 



D 



Diagram of the steps in crossing-over. A, an allelomorphic pair of chromo- 

 somes, with genes represented as soHd or open circles ; B, synapsis, or the con- 

 tact of homologous chromosomes ; C, breakage of chromosomes at the point of 

 contact ; D, reassembly of chromosome fragments, resulting in a cross-over of 

 genes, making a new combination. 



sugary grains (s). Thus, when pure colored-starchy corn (CCSS) is 

 crossed with pure colorless-wrinkled corn (cess), the resulting hybrid 

 will be colored-starchy like the dominant parent in appearance but 

 with the genotypic formula of CcSs. When in turn these hybrids are 

 back-crossed with the recessive parent (cess), in order to reveal the 

 different kinds of offspring which they are capable of producing, the 

 expected result, if there is independent assortment, would be the ratio 

 of 1 CS :lCs: IcSilcs, as shown below. 



In an actual experiment, however, when the hybrid was back-crossed 

 to the recessive parent, the offspring were phenotypically 4032 

 CS : 149 Cs : 152 cS : 4035 cs. This is approximately the ratio of 

 48 : 2 : 2 : 48, instead of the expected 1:1:1:1. The explanation 

 of this result is that out of a total of 8368 cases there were 8067 

 instances in which the characters CS and cs, that entered into the 



