As humans, we have receptors for two kinds of beta adrenergic compounds such as catecholamines. Cardiac muscle cells have beta 1 receptors that promote increased heart rate. Some drugs that slow heart rate are called beta blockers. Smooth muscle cells, however, have beta 2 receptors which mediate smooth muscle contractions. Blockers of these effects are sometimes used to treat asthma.

The description above illustrates what?

just because a drug acts on one type of receptor does not mean that it will act on another type.

Explanation:

Based on the scenario being described within the question it can be said that this description best illustrates that just because a drug acts on one type of receptor does not mean that it will act on another type. That is because there are a variety of different receptors each of which target different areas and have different functions, therefore a drug that performs a specific function cannot be used on all receptors.

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.