Genetically, double eyelids belong to dominant genes, while single eyelids belong to recessive genes. In the world distribution, single eyelid is the characteristic of a small number of people in Asia, accounting for about 10% of the world population, while Europeans, Americans and South Asians are basically double eyelids; It is more reliable to say that the single eyelid is the legacy of the Hu nationality.

Single eyelid and double eyelid genes

Double eyelids refer to the skin with shallow grooves above the eyelid margin. When the eyes are open, the skin under this shallow groove moves up, and the skin above this groove sags and forms an obvious shallow groove.

Single eyelid and double eyelid are a pair of relative traits in genetics, in which double eyelid is controlled by dominant gene (a) and single eyelid is controlled by recessive gene (a). For example, a pair of Aa parents, the gene distribution of their offspring is shown in the following figure:

This means that in people with a certain number of single eyelids and double eyelids, the number of single eyelid traits will gradually decrease, and it will only appear when two recessive genes meet.

Biological influence

According to the characteristics of single eyelid and double eyelid, from the biological point of view, in cold environment, the fat layer of single eyelid is thicker (almost twice that of double eyelid), which can protect the eyeball and resist the cold better than double eyelid, while the shallow groove of double eyelid is easier to freeze and be attacked by cold wind, which may even make the eyes hard to open in severe cases, but in warm or hot environment, the two have little effect. Thus, as far as biological survival is concerned, a single eyelid.

However, in modern aesthetics, double eyelids with big eyes are more popular than single eyelids with small eyes, especially a pair of obvious double eyelids, which has become the pursuit of women and even risked being double eyelid surgery.

Single eyelid origin

At present, people with single eyelids are concentrated in all parts of the world, and single eyelids are recessive symptoms, so it just illustrates one point? The single eyelid is probably a feature left over from a species.

According to the population distribution of single eyelid, there is a reliable saying that single eyelid is a feature that evolved from ancient Hu people (northern nomadic people) and was left by Hu people's lineage.

In the history of China, the nomadic people in the north have merged with the Han nationality many times, or rather, the Han nationality assimilated the integrated conference semifinals; But some of their genes are inherited, and single eyelid is an obvious feature. Judging from the distribution of single and double eyelids in China, the closer to the south, the lower the probability of single eyelid, which just shows that single eyelid migrated from the north to the south.

Koreans have a single eyelid rate of over 80%. This is because Koreans have the blood of basil, and basil is also a nomadic people in ancient north, so many Koreans have the gene of single eyelid; For Siberian aborigines, the proportion of single eyelid is close to 100%, and the proportion of Mongolians is also very high.

Single eyelid is inherited by a genetic mutation of northern nomadic people. Other pure-blooded people don't have this gene. Due to the flow of population, some single eyelid genes will flow to the whole world, but single eyelid is extremely rare in Europe, America, South Asia and black people.

Future evolutionary speculation

For modern people, there are not many people who live in cold outdoor for a long time. In addition, the biological advantages of single eyelid can be ignored, but people pay more attention to aesthetics, and double eyelids are more popular.

Nowadays, human beings communicate frequently, and single eyelid is a recessive gene, which is much lower than double eyelid. Therefore, there will be fewer and fewer single eyelids in the future population, but the overall single eyelid base is still very large (10% is 750 million people), so it will take a long time to minimize the number of single eyelids, but the single eyelid gene will still exist in the population. Once two recessive genes meet, the characteristics of single eyelid will appear.