A scientist is studying the inheritance of two characteristics in plants: red flowers (RR and Rr) , which are dominant to yellow flowers (rr) , and green leaves (GG and Gg) , which are dominant to yellow leaves (gg) . She crosses a double heterozygous (RrGg) with a double recessive (rrgg) , and expect to see a 1:1:1:1 ratio in the offspring. Which phenomenon would you hypothesize accounts for the pattern you see?