Imprinting is a process that happens beyond genetics, and is therefore called epigenetic. An epigenetic event can change the expression of a cell’s gene without changing the basic genetic code. During imprinting, one or more molecules are added to the gene, acting as a stop code. These molecules tag the DNA and prevent it from being transcribed into proteins, therefore blocking the expression of that gene.
Here’s some motivation: there’s a surprise at the end. You’ll find yourself starting the book all over again.