Many people believe that a mutation will always result in the formation of a deadly tumor, or the development of a major
body defect.
Explain why this is incorrect.

The idea that mutations can only cause negative alterations -such deadly tumor or a major  body defect- is wrong, because mutations can induce beneficial changes to organisms or be practically imperceptible, without physically or functionally affecting individuals.

Explanation:

Mutations consist of changes in the genome of a species, i.e. an alteration in the DNA containing the genetic information, which may or may not be passed on to the offspring.

Mutations result in genetic diseases, which can lead to physical and functional defects, as well as a predisposition to chronic disease. The appearance of malignant tumours, such as cancer, is another negative consequence of mutations.

Changes due to a mutation are not always negative, as mutations are the basis for genetic variability in species.

First of all it must be understood that mutations linked to reproductive cells are inheritable. This ability of individuals to pass on information to their offspring implies variations in subsequent generations. These variations may be observed more rapidly in species with short reproductive cycles. A classic example of this is bacterial resistance to antibiotics, an inheritable trait.

Thanks to mutations, certain variations can be observed in populations which -although they affect their appearance - do not involve major deformities or functional alterations. In addition, a mutation can produce changes that consist of an evolutionary advantage that allows the adaptation and survival of a species to changes in its environment, over time.

Learn more:

Mutation as mechanism for evolutionary change link