This process, called homologous recombination, is usually very rare, which is where the viral enzymes come in.

FORBES: Putting Evolution to Work

They trick cells into treating the added DNA as its own, greatly increasing the chance of homologous recombination.

FORBES: Putting Evolution to Work

But are these genes unique to the fly, or do homologous genes determine the development of a human embryo?

ECONOMIST: Genetic science

When geneticists replaced one of the mutant fly genes by its homologous mouse gene the fly developed normal flies' eyes and not mouse eyes.

ECONOMIST: Genetic science

Similarly, a mouse which had its own gene controlling development of the eye replaced by the homologous fly gene developed a normal mouse eye.

ECONOMIST: Genetic science