Q.Explain, with an example, how transgenic mice carrying a human gene have been used to study how a particular gene causes a disease such as cancer.

[!TLDR] A transgenic mouse carrying a human oncogene reliably develops the associated disease/symptoms, giving researchers a living model in which to study how that gene causes disease and to test treatments.

[!ANSWER] By introducing a specific human gene associated with a disease -- for instance an oncogene linked to a particular cancer -- into a mouse embryo, researchers create a transgenic mouse strain that reliably develops that disease or shows measurable related symptoms. This living disease model lets researchers directly study the disease process caused by that gene, and test potential treatments, in ways not practical or ethical to study directly in human patients.

Studying exactly how a defective or abnormally active gene contributes to a human disease is very difficult to do directly in human patients, both because it would often require invasive procedures or experimental treatments that cannot ethically be tested on people, and because the disease process typically needs to be observed and manipulated over the course of an animal's whole lifespan or through controlled breeding across generations, neither of which is practical or acceptable in human research.

Transgenic mice solve this problem by allowing a specific human disease-related gene to be introduced directly into a mouse's genome. If, for example, an oncogene -- a gene known to be linked to the development of a particular type of cancer in humans -- is introduced into a mouse embryo in this way, the resulting transgenic mouse strain can be bred to reliably develop that same cancer, or clearly measurable symptoms and biological changes associated with it, as the mouse grows.

Because this disease process now occurs reliably and reproducibly in a living organism that is far easier, faster, and more ethically acceptable to study and experiment on than a human patient, researchers can use this transgenic mouse model to investigate exactly how that gene drives the disease process at a cellular and molecular level, and can also use these mice to test whether candidate drugs or other treatments are effective at slowing, stopping, or reversing the disease, before any such treatment would ever be considered for a human clinical trial.

[!ANSWER] A transgenic mouse carrying a human oncogene provides a reliable, reproducible, living model of the human cancer that gene is associated with, letting researchers study the disease mechanism and test potential treatments directly, in a way that would not be possible or ethical to do in human patients.

Explain first why studying a disease gene directly in humans is impractical/unethical, then explain how the transgenic mouse model provides a workable substitute.

Do not claim the transgenic mouse itself becomes a treatment or cure -- it is a RESEARCH MODEL used to study the disease and test candidate treatments, not a therapy in itself.