One might very well ask why the resultant esrog is not passul, just as the result of grafting two species is passul. Rabbi Shlomo Zalman Auerbach, zt”l (cited in Kashrus Arbaas Haminim, p. 182), explains that the branch of a lemon tree is capable of producing fruit by itself. Therefore, when it is grafted to an esrog tree, two viable species have been joined resulting in a third species – a hybrid fruit which is not considered an esrog. However, the pollen of a lemon flower cannot grow into a fruit by itself. It only provides genetic material, allowing the pistil of the esrog to develop into a fruit. Therefore, what results from it is considered a kosher esrog fruit, even if there are some traces of lemon.
According to this explanation, it would seem that esrog farmers might be allowed to make use of the latest techniques in agricultural biotechnology, in which chromosomes of a certain species are isolated and injected into a different species, giving it traits of the first species. Since this genetic material could not develop into a plant on its own, and is only capable of altering its new host, the result may very well be a kosher esrog tree with the longevity and durability of a lemon tree. However, the Chazon Ish (Kilayim 2:15) notes that even in such a case, the esrog will only be kosher if it has both the outer and inner appearance of a pure esrog.