Genetics is a direct or indirect influence on much if not most tumour production. Some individuals have specific cancer-producing genes. In the Bernese Mountain Dog for example, 60 percent of individuals die as a result of tumours, often lymphomas and bone tumours.
In other individuals or breeds, the genetic link is more complicated. Individuals may inherit a cancer-producing gene but hopefully also a vital cancer-suppressing gene. It gets more complicated. Some pets may also inherit a gene that suppresses the cancer-suppressing gene! Regardless of what genes are inherited, the genes are turned on and off by environmental factors. Some environmental causes of malignant tumours are known, including ultraviolet light (skin tumours), radiation (thyroid tumours and leukemia) and various chemicals. Other environmental factors, such as diet and "stress" are far more difficult to define. This is what makes the development of tumours such an unpredictable phenomenon. In some cats minor trauma and certain virus infections may predispose to cancer although this is not known to occur in dogs.