Final answer:
A transversion substitution occurs when a pyrimidine replaces a pyrimidine or a purine replaces a purine.
Step-by-step explanation:
When a pyrimidine replaces a pyrimidine or a purine replaces a purine, a transversion substitution has occurred. This type of substitution involves a purine being replaced by a pyrimidine or vice versa.
For example, adenine, a purine, may be replaced by cytosine, a pyrimidine.