What is a knock out model and what has been found using knock out models? what is a knock in model?

Knock out: an animal model in which researchers have inactivated an existing gene by replacing or disrupting it with an artificial piece of DNA. it has been found that most genes are pleiotropic; they are expressed in different tissues, in different ways and at different times and that knock outs of many genes in the mouse genome can be compensated for. knock in: an animal model in which a gene sequence of interest is altered by one-for-one substitution with a transgene, or by adding gene sequences that are not found within the locus.