THE BREAKDOWN OF CONTROL 



Fig. 64. — Breakdown of the genetic determination of incompatibility in Petunia. 

 P. violacca plants of the constitution SjSo set seed from only about i per cent of flowers 

 which have been self-fertilized (indicated by size of black square relative to the 

 white one containing it). S1S2 plants recovered in the F2 of the cross with the self- 

 compatible P. axillaris set seed from about 25 per cent of flow^ers after self-pollination 

 and S^So plants recovered in the backcross to P. violacea are intermediate in behaviour. 

 Thus the addition of genes from P. axillaris causes progressive breakdown of the 

 incompatibility mechanism of P. violacca. It also causes some change in the operation 

 of the mechanism which is left, as pollination of the backcross or F.^ plants by 

 P. violacea gives a greater set of seed than does selfmg, with which the cross is 

 identical in respect of Sj and S.^ (based on Mather, 1943). 



Elements of Genetics 



257 



