Why might parents who don't show the trait of albinism have children who do?

This trait is rare when it occurs. Some genes depend on other genes to be expressed, so in most cases a trait is denied an expression. But its still there, so it passes down to generation until it gets expressed.

Explanation:

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.