The main difference between mono eyelids and double eyelids is the appearance of the eyes, which appear larger in patients with double eyelids, and is mainly caused by the genes that determine the phenotype of double eyelids. Double eyelids and single eyelids are just one of the different biological phenotypes of the face, and both are normal and not pathologically altered. However, in double eyelids, the eyes look larger than in single eyelids due to the formation of a skin fold in the upper eyelid near the lid margin. Double eyelids and single eyelids are genetically determined. The gene that determines the phenotype of double eyelids is a dominant gene, and people with single eyelids can have double eyelids through cosmetic ophthalmologic surgery. It is recommended that patients with eyelid anomalies seek medical attention to evaluate their condition and follow their doctor’s instructions.