You inherit your genes in pairs: one of each pair comes from each of your parents. Each pair of genes provides two versions of the same instruction - these are called alleles. Sometimes, the product of one version can mask that of the other version. For example, someone with brown eyes may also have the instruction for blue eyes. But the effect of the brown-eye gene masks the effect of the blue-eye gene. The effect of genes masked in one generation can sometimes show up in the next.