Gene knockout is a phenomenon in which both the copies of a gene are modified and converted into their inactive forms. This happens in the case of diploid organisms. Gene knockout is done by manipulating the DNA sequence of a gene and this change may either suppress the function of the gene or inactivate the gene.
This phenomenon is used in order to study the function of the specific gene is performing. It also becomes possible to control the effect the gene produces and to study the contribution of a gene in causing disease. The methods through which the gene is knocked out are gene silencing, conditional knockout, gene editing etc.