Eukaryotic genes consist of alternating blocks of protein-coding sequences called exons and noncoding sequences called introns.

Exon recombination is a process where the exons from the same or different genes recombine to produce novel combinations of exon-intron sequences, which might evolve into new genes.

The exon recombination can be mediated by non-homologous recombination or retro-transposition.

During non-homologous or illegitimate recombination, DNA strands with no sequence similarity can recombine and produce a novel gene structure.

Chronic granulomatous disease - a genetic disorder of the immune system is a good example. Many patients with this disease have a rare mutation in the phox gene coding for the enzyme NADPH oxidase.

The wild type NADPH oxidase produces reactive oxygen species such as superoxide upon infection and aids in phagocyte-mediated killing of the infectious agents.

In rare cases, the non-homologous sequences between exons 8 and 11 of the phox gene recombine, causing duplication of exons 9 and 10. This leads to the reduced activity of the enzyme NADPH oxidase. The impaired enzyme fails to generate enough reactive oxygen species to clear the bacterial infection resulting in granuloma or aggregation of macrophages and associated immune cells. 

Exon recombination due to retrotransposition can best explain the evolution of the Jingwei gene in African Drosophila.

Around 2.5 million years ago, a portion of the Adh mRNA reverse-transcribed into DNA and recombined with exons of the Ynd gene to give rise to a new gene called Jingwei.

The Ynd gene is responsible for the development of testes in drosophila and the Adh gene encodes alcohol dehydrogenase. The new gene Jingwei encodes a protein with two domains - one derived from Ynd and the other from Adh.

Hence, the protein Jingwei is expressed in the drosophila testis and is responsible for hormone and pheromone biosynthesis.