Why is there a difference between single eyelid and double eyelid?

The difference between a single eyelid and a double eyelid is determined by genes.

Genetically, the parental traits of the first generation of purebred hybrids with a pair of relative traits are called dominant traits, and the genes controlling the dominant traits are called dominant genes; Parental traits that are not revealed are called recessive traits, and genes that control recessive traits are called recessive genes. Dominant genes are usually represented by capital letters, while recessive genes are usually represented by lowercase letters.

Single eyelids are also called single eyelids, and those without folds are called single eyelids, commonly known as single eyelids. Double eyelid means that the skin of the upper eyelid has a shallow groove above the eyelid margin. When you open your eyes, the skin under this groove moves upward, while the skin above this groove hangs loosely at the double eyelid groove and folds down into a horizontal fold, which is also commonly known as double eyelid.

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In ancient times, a single eyelid has always been a beauty. Although the men and women in the portrait have both single eyelids and double eyelids, the beautiful women in the pictures of ladies in past dynasties are all single eyelids without exception. Why is this? The reason is simple: portrait painting must be faithful to the facts, but there is no such restriction on painting beautiful women. As long as you follow the agreed aesthetics, you can draw your own beauty.

The aesthetics of the past dynasties are not immutable. For example, the Tang people advocated beauty and fullness, while the Ming and Qing people advocated delicacy and gentleness, but the only constant thing was the preference for single eyelids. In painting, from the oldest existing figure painting ── A Historical Picture of Women by Gu Kaizhi in the Eastern Jin Dynasty to the late Qing Dynasty and the early Republic of China, and even before the Anti-Japanese War, the preference for single eyelid has never changed.

The monotonous single eyelid in the beauty picture may be realistic in the Jin and Tang Dynasties. Looking up, you can see that both men and women have single eyelids. Naturally, it is impossible for all painters to have double eyelids, just as western painters do not paint westerners with single eyelids.

But after the Song Dynasty, painting a beauty as a single eyelid became a procedure. It can be seen that ancient people loved single eyelids. In short, before China people's aesthetics were surpassed by western aesthetics, China people had their own standards for the identification of beauty.

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Baidu Encyclopedia-Single Eyelid (Hidden Eyelid)