A few genetic tools exist, but a fairly common method when I was studying it was to use a retro-virus containing failed copy of the gene, and insert it into very young, often single cell embryos, so it would infect all the cells in the target animal, and damage the target gene, either by removing the stretch of DNA it was on, or by changing the portion of the DNA strand which tells the body there is a protein to build here, which I believe is called the promoter.