XI COUPLING AND REPULSION 121 



+ I niUl + 7 BblU I bbLL + 7 bbLU^c) bbll 

 + 7 Bbll + 7 bbLl 



1 5 purple 1 5 red 49 red 



round long round 



The Fg generation should consist of the 4 classes, 

 purple long, purple round, red long, and red round 

 in the proportions 177 : 15 : 15 : 49. The sum 

 total of the plants bred in this way was 6952. On 

 the hypothesis outlined above we should expect it 

 therefore to consist of 48 i 4 purple longs, 408 purple 

 rounds, 408 red long, and 1332 red rounds. A 

 glance at the scheme given on p. 120 shows how 

 closely this expectation tallies with the experimental 

 results. Corroborative evidence was also obtained 

 by crossing red round plants, with the pollen of the 

 F^. This method of crossing with the double re- 

 cessive affords a direct test of the nature of the 

 gametic series produced in the ratio 7:1:1: 7, 

 and the actual numbers obtained, viz. 50 purple 

 longs, 7 purple rounds, 8 red longs, and 47 red 

 rounds again, tally closely with expectation. 



A sweet- pea heterozygous for B and L may 

 arise in two different ways — either through the 

 union of a BL gamete with a hi one, or else by the 

 union of Bl with bL. The former case, we have 

 just discussed : let us now consider what happens 

 in the latter. When a purple round is crossed with 

 a red long the F^ plants are long-pollened purples 

 as before. The F., generation thus consists of the 

 4 possible classes of plants as before, and as before 

 the purples are to the reds as 3 : i, as also are the 

 longs to the rounds. But the proportion in which 

 the four classes occur now shows a marked difference. 



