Question

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

Answer 1

It is easier to establish a pure-breeding population for recessive genes.

Explanation:

Depending on exactly what your question means, it is easier to establish a true-breeding population for a recessive trait.

That is because "what you see is what you get."

There are no hidden genes waiting to show up in the next generation.

In one single generation, you can delete all the individuals with the dominant gene just by looking at them for the trait.

The remaining population is is all recessive, which you can easily see just by looking at them.
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But if you are trying to establish a homozygous dominant population, you will have a lot more trouble.

The dominant gene masks the presence of the recessive gene, so there is no visible difference between homozygous dominant and heterozygous individuals.

`Aa` individuals usually look just the same as `A A` individuals, so it is hard to delete the recessive gene.

You can cull the homozygous recessive individuals (`aa`), but that deesn't get the recessive genes that are masked in heterozygous individuals.