Which is easier to establish in a pure-breeding population, a dominant or a recessive gene?

For a recessive trait, it is simpler to establish a true-breeding population, depending on the specifics of your question.

For the simple reason that "what you see is what you get."

There aren't any undiscovered genes that will surface in a subsequent generation.

By simply observing them for the characteristic, you can eliminate every person carrying the dominant gene in a single generation.

You can tell a lot about the remaining population just by looking at them—they are all recessive.

However, the challenges involved in establishing a homozygous dominant population will be much greater.

The homozygous dominant and heterozygous individuals do not exhibit any discernible differences because the dominant gene obscures the recessive gene's existence.

In a pure-breeding population, it is easier to establish a dominant gene compared to a recessive gene.