Um, the answer is they wouldn't be expressed because of a new new way in which the cell regulates itself known as RNAi. The inserted gene would be silenced as was observed in the 1980s when attempts to insert a transgene on a vector to flowering plants resulted in silencing of both the inserted gene and the endogenous gene.
Its a defense mechanism against both retroviruses and normal DNA viruses which attmept to insert their foreign genome into the genome of the plant so that the viral genome can replicate and produce more of themselves during the lysogenic phase.
Either through post transcriptional or transcriptional silencing the gene is not expressed.
And dttom, you can still use a p element to recombine the transgene using non homologous recombination, its just that the frequency of recombination would be very low.
