Usually transgenic animals in which specific genes can be knocked out (or deleted) only in certain cells or at certain times under experimental control in an attempt to identify what effect that gene has in the life of the organism. The mouse is unique among mammals with regard to the degree to which its genome is amenable to such genetic manipulation, and the production of gene knockouts has become a routine task for identifying genes responsible for a range of diseases (e.g., Huntington’s disease). Thus, mice are good for reverse genetics, but not forward genetics. Progress in producing mice with conditional knockouts at certain stages of development when genes are inactive or that can be turned off and on using drugs and environmental changes has proved more difficult to achieve. In contrast, knock-in mice are generated to study the exogenous expression of protein through the insertion of a transgene at a selected locus on the chromosome.
See Chromosome, Forward genetics, Huntington’s disease (or chorea), Otoliths, Reverse genetics, Transgene