Gene Knock In: In molecular cloning and biology, gene knock in (KI) refers to a genetic engineering method that involves the one-for-one substitution of DNA sequence information in a genetic locus or the insertion of sequence information not found within the locus. Typically, this is done in mice since the technology for this process is more refined and there is a high degree of shared sequence complexity between mice and humans. In summary, Gene KI is the process of targeted insertion of an exogenous gene at a specific locus in the genome. It is used to modify the function of an endogenous gene in order to study the detailed mutations of small nucleotide in human disease. A common use of knock-in technology is for the creation of disease models.
Transgenic: Transgenic is a term that describes an organism containing genes from another organism put into its genome through recombinant DNA techniques. An example of its usage is the term transgenic organism. A transgenic organism is one that contains a gene or genes which have been artificially inserted instead of the organism acquiring them through reproduction. It is understood that the foreign genes are in the transgenic animal's germ-cell DNA and so can be transmitted from one generation to the next.
Most transgenic organisms are generated in the laboratory for research purposes. Transgenic organisms are used in research to help determine the function of the inserted gene, while in industry they are used to produce a desired substance. Transgenic organisms have been developed for commercial purposes. Perhaps the most famous examples are food crops like soy and corn that have been genetically modified for pest and herbicide resistance. These crops are widely known as "GMOs" (genetically modified organisms).
Difference between knock in and transgenic: The main difference between knock in technology and traditional transgenic techniques is that knock in is targeted, meaning the desired gene is inserted into a specific locus in the target genome via homologous recombination, and is thus a "targeted" insertion. By contrast, transgenic models use random integration: the desired gene could end up anywhere in the host genome. The desired gene might be placed under its own (strong) promoter, leading to high levels of expression, which can be good for disease models, leading to an earlier or more robust phenotype. But the tradeoff is that it's not as faithful a model of the human disease. One possible issue is that the expression pattern might be all wrong.
In either knock in or transgenic models, the introduced gene can be from the same species or a different species. A few web hits defining transgenic as meaning trans-species but that is not how scientists use the term. There is no question that researchers in the prion disease field take the terminology "transgenic" to include same-species random integration. Just as transgenesis doesn't necessarily involve a different species, the gene knock in doesn't necessarily imply the gene is from the same species. A search for "knock in human gene in mice" did reveal researches doing knock in of human genes into mice.
1. Doyle et al. The Construction of Transgenic and Gene Knockout/Knockin Mouse Models of Human Disease. Transgenic Res. 2012; 21(2):327–349.
2. Eric Vallabh Minikel. The difference between knock-in and transgenic mice. CureFFI.org. Nov 13, 2012.