(2) Chromosomes in somatic cells exist in pairs, and in the process of cell division to form sperm and egg cells, chromosomes will be reduced by half. Moreover, not any half, but one in each pair of chromosomes enters sperm and egg cells. A is the gene that controls double eyelids, and A is the gene that controls single eyelids. This person gave birth to a child with a single eyelid, and the genetic composition is aa, so it can be determined that this person's genetic composition is Aa or AA.
(3) When a pair of genes controlling a certain trait, one is dominant and the other is recessive, only the trait controlled by the dominant gene will be displayed; This person's genetic component is Aa, and the trait controlled by recessive gene A is not expressed, so this person's trait is normal skin color. The closer the blood relationship is, the closer the genetic genes are, and the greater the chance that the children born after marriage will get genetic diseases. So consanguineous marriage is forbidden.
So the answer is: (1) protein and DNA；; (2)2; Aa or aa; (3) normal skin color; It is forbidden to marry close relatives.