Gene Silencing
When scientists began to work with transgenic plants, they found that just because the transgene is passed on to offspring doesn’t mean the offspring will be able to actually turn the gene on and have it influence a trait. This is because of something called gene silencing.
Plants have a natural ability to silence their genes so that they will be present but not turned on and expressed. Apparently this natural ability allows the plant to control the expression of extra genes that may appear in their chormosomes because of mistakes in replication, invading viruses, or some other poorly understood processes. Scientists knew that certain genes that control corn kernel colors were silenced in the plant on occasion even before they worked with genetically engineered plants.
The occurrence of gene silencing in transgenes, however, occurs at a high enough rate that we now know that it will be a problem that needs to be better understood.
During transformation, one gene copy may insert into a chromosome, as in the picture above, or several gene copies may insert, as in the picture below. Events where multiple gene copies have inserted are suspected to have a higher probability of gene silencing. (Image credit: P. Hain)
Often genes are not silenced in the original transgenic plant, but will be silenced in its progeny. The plant cell somehow recognizes the new genes and then chemically modifies the DNA so it cannot be read for protein synthesis. It is believed that silencing depends upon where the gene inserts and how many copies of the gene insert at that position of the chromosome. For example, the insertion of more gene copies can result in a higher probability of silencing. Therefore certain events will be more prone to gene silencing than others.
(Image credit: P. Hain)