THE PHYSICAL BASIS OF INHERITANCE 



293 



Boveri hypothesis had speculated that two or more genes might reside 

 within a single chromosome and had pointed out that genes so located 

 should (from all evidence then available) remain permanently associated 

 and invariably be inherited together. This idea was based upon theoretical 

 considerations, not upon any actually known instances. 



And then, in 1906, Bateson and Punnett encountered a supposed 

 dihybrid cross in which neither independent assortment nor the per- 

 manent association of two genes would explain the F 2 progeny. They 

 crossed a sweet pea that was homozygous for purple flowers and long 



Color Blind Normal Normal Color Blind 



00 



Blind 



00 



f 



J^ 



7 T 



^ 



? ? O 



01 00 00 01 01 



Fig. 19.1. Diagram of the sex-linked inheritance of color blindness in man. C represents 

 the dominant gene for normal vision, c the recessive gene for color blindness. Note that only 

 the X chromosomes carry Core; the Y chromosomes (shaded) are represented as empty. 



pollen grains with one that was homozygous for red flowers and round 

 pollen grains. The Fi had purple flowers and long pollen grains, showing 

 that purple was dominant to red and that long was dominant to round. 

 The F 2 generation, however, showed a marked deviation from that 

 expected on the basis of independent assortment. The actual ratios and 

 those expected on the basis of independent assortment in the 6,952 F 2 

 individuals that were obtained are tabulated below: 



