Genetics and blue eyes
@another 'Homo sapien' is basically correct, and what they were getting at, is that in just the F1 generation, you're right, any recessive traits are masked. However, if you continue this onward, in subsequent breeding, recessive traits are unmasked. (This is basically why inbreeding is a bad thing).

If we use the traditional Punnet square method for determining genotype:

if A is a Brown-eye dominant allele, but a is a blue-eye recessive allele:

F1:



You can see that a single recessive allele is now present in 50% of the offspring of a given pair of parents.

Now, if in F2, we assume that 2 of those recessive offspring were to mate, the recessive gene would be unmasked in 1 quarter of the offspring (statistically speaking), and 50% would be carriers.

F2:



Now it gets a little more complicated. Our double recessive F2 member has blue eyes phenotypically, they aren't just a recessive allele carrier.

F3:

So, if in F3, our aa mates with another aa, (ignoring spontaneous mutation for simplicity) it should be obvious that all of their offspring in F3 would have blue eyes (and I can't quickly find a Punnett square in Google showing this).

However, if our aa now mates with an AA (homozygous dominant brown), we will get 100% offspring with a heterozygous Aa genotype - in other words, all blue eye carriers, but with brown eyes phenotypically.

If aa mates with a heterozygous individual though (Aa), we have the following:



And thus, 50% blue eye individuals, with 50% brown heterozygous dominant. And so on and so forth, ad infinitum.

Hopefully that explains how a recessive allele can go to fixation despite only occurring once.