In several plants, such as tobacco, primrose, and red clover, combinations of alleles in eggs and pollen have been found to influence the reproductive compatibility of the plants. Homozygous combinations, such as S¹ S¹, do not develop because S¹ pollen is not effective on S¹ stigmas. However, S¹ pollen is effective on S² S³ stigmas. What progeny might be expected from the following crosses (seed parent written first): 

(a) S¹ S² × S² S³; 

(b) S¹ S² × S³ S⁴;
(c) S⁴ S⁵ × S⁴ S⁵; and 

(d) S³ S⁴ × S⁵ S⁶? .