Regardless of experimental system, genetical study has made its position on the core of our research. It helps us to identify the function of a specific gene or vice versa. First we may find our desired phenotype and then go for responsible gene, more precisely mutational site. On the other hand, we may mutate one particular gene and observe the corresponding phenotype. As we can see the TWO WAY genetic approaches are open for us. The earlier one is known as Reverse genetics and latter one as Forward genetics. The providing picture will help to understand the basic concept of forward and reverse genetics in a more lucid way.
For reverse genetics, insertion of foreing DNA such as transposable element or T-DNA causes disruption of gene function. Between these two types of insertional mutation, T-DNA is much more preferable compared to transposable element as it's physically and chemically more stable in next several generations. In addition, advancement of Agrobacterium mediated transformation has made T-DNA a suitable choice for whole genome mutagenesis.
But T-DNA insertional mutation has diverse consequences based on both T-DNA and insertional site. So, T-DNA insertional mutation may give total knock out phenotype or overexpression. It's the interesting part. Follow the figure below:
T-DNA insertion may happen on promoter regions or coding regions. As a consequence of T-DNA insertional position, there are are three possible outcomes. No expression (knock-out), reduced expression (knock-down) and not completely null in terms of expression (knock-about). Interestingly, if inserted T-DNA construct contains sequences like promoter regions of 35S cauliflower mosaic virus, it increases the expression of the following gene. That's why considered as knock-on. Sometimes mutile T-DNA insertion happens in one gene of one plant and we call it knock-knock. The severe kind of insertional mutation caused chromosomal rearrangement. This rearrangemental event is regarded as knock-worst.
If you are interested about this knockology post, you should read the following article for details.