What are knockout animals?

Answer 1

Animals that have knockout genes. Knock out gene:

A gene knockout (abbreviation: KO) is a genetic technique in which an organism is engineered to carry genes that have been made inoperative (have been "knocked out" of the organism). This is done for research purposes. Also known as knockout organisms or simply knockouts, they are used in learning about a gene that has been sequenced, but which has an unknown or incompletely known function. Researchers draw inferences from the difference between the knockout organism and normal individuals. The term also refers to the process of creating such an organism, as in "knocking out" a gene. The technique is essentially the opposite of a Gene Knock-in. Knockout is often abbreviated as KO. Knocking out two genes simultaneously in an organism is known as a double knockout (DKO). Similarly the terms triple knockout (TKO) and quadruple knockouts (QKO) are used to descripe 3 or 4 knocked out genes, respectively.

Source: http://en.wikipedia.org/wiki/Gene_knockout

Answer 2

Knockout animals are genetically engineered animals that have had a specific gene deactivated or "knocked out" to study the function of that gene. These animals are used in research to understand the role of certain genes in disease development, physiology, and behavior.