For a very simple example, if the genes controlling single eyelid and double eyelid are big A and small A respectively, then the combination aa and aa are both double eyelids, and Aa is single eyelid. According to this ratio, the ratio of double eyelids to single eyelids should be 3: 1. If the mother in a family has a single eyelid, then she is the aa gene, and the father may be the Aa or Aa gene, but if both their children have single eyelids, it means that the father must be AA, because the mother can only provide the small A gene, and the child is the single eyelid, that is, the AA gene, which means that your father must provide you with the small A gene. He has double eyelids, so he has the big A gene. So he is the dominant gene of A, and the recessive gene of A is Aa. As mentioned earlier, the big A is visible and the small A is invisible, so when the big A and A are combined, they show the characteristics of visible genes, and with the invisible gene small A, they show double eyelids. In other words, parents have single eyelids, and children must have single eyelids.