;.J96 W. Batesoll. 



Gametic Coupling of characters belonging to distinct 

 allelomorphic pairs. 



When all the factors belonging to all the allelomorphic pairs are 

 distributed independently among- the gametes, the number of each 

 kind of gamete and the results of their several unions can be calcu- 

 lated by the simple rules of probability. Sometimes however there 

 is evidence of a linking or coupling between distinct characters. 

 When such a coupling is complete, the two characters of course can" 

 be treated as a single allelomorph. The importance of this kind of 

 coupling was remarked on by Correns early in the history of Men- 

 delian research. Complete coupling has thus far been most commonly 

 met witli among characters of similar physiological nature, e. g., 

 flower-colour with axil-colour, &c. As an illustration of coupling 

 between characters distinct in nature may be mentioned the asso- 

 ciation in the Cupid (dwarf) Sweet Pea of dwarf liabit and procum- 

 bency, an association not found in Pisum. 



But besides this simpler phenomenon of complete coupling we 

 now know that the usual ratios are liable to disturbance by a partial 

 coupling between distinct characters. Such examples have not yet 

 been fully studied. The best known is that of the i)ollen-shape with 

 flower- colour in the Sweet Pea. When White with long pollen-grains 

 is crossed with White having round pollen-grains, Fj, as already 

 seen, is often purple, and gives in Fo, 27 Purple : 9 red : 28 white. As 

 regards pollen, F, is long, and Fg as a whole contains 3 long : 1 white. 

 The white also taken alone are 3 long : 1 round. The purples however, 

 show a great preponderance of longs, about 12 to 1 round, while 

 among the reds the rounds are in excess over the longs, being about 

 3 to 1. Evidently the abnormal distribution in some way depends 

 on a coupling of B, the blue determiner, with long pollen. A close 

 approach to the observed F2 numbers would be given by a system 

 in which each 16 gametes were composed thus: 



7 AB4- 1 aB + 1 Ab + 7 ab 

 where A is long pollen and a round pollen. 



Purple Red White 



The diagram on page 389 exhibits the distribution of the factors, 

 and the consequences of their unions. 



