A plant species X with n = 5 was crossed with a related species Y with n = 7. The F₁ hybrid produced only a few pollen grains, which were used to fertilize the ovules of species Y. A few plants were produced from this cross, and all had 19 chromosomes. Following self-fertilization, the F₁ hybrids produced a few F₂ plants, each with 24 chromosomes. These plants were phenotypically different from either of the original species and were highly fertile. Explain the sequence of events that produced these fertile F₂ hybrids.