When a particular mutagen identified by the Ames test is injected into mice, it causes the appearance of many tumors, showing that this substance is carcinogenic. When cells from these tumors are injected into other mice not exposed to the mutagen, almost all of the new mice develop tumors. However, when mice carrying mutagen-induced tumors are mated to unexposed mice, virtually all of the progeny are tumor free. Why can the tumor be transferred horizontally (by injecting cells) but not vertically (from one generation to the next)

because the mutagen induced tumor causing mutations in somatic cells, not in gamete producing cells in the germline.

Explanation:

The gene can be transferred either horizontally or by vertically.

The above scenario is related to horizontally transferred of mutagen by tumor cells to the non-exposed mutagen mice. By transfer of mutagen cells, tumor develop the mice. When mutagen-induced tumor mice are mated with unexposed mice, there will be no development of tumor into the progeny.

It clearly shows that mutation occurs in the somatic cells only due to mutagen and it does not affect the gamete producing cells in the germline.