Which of these characteristics is fixed by inherited alleles alone?

Explanation

Blood group is settled by the alleles a person receives at fertilisation and stays the same whatever that person eats, trains for or lives through, so nothing in a lifetime moves it. Skin darkening after a summer outdoors is the closest rival, since inherited alleles do set how strongly a person responds, but the darkening itself comes and goes with the sunlight received and so is not fixed by alleles on their own.