What is the problem with people whose little toe nails split in two? Is your nail like this too?

Produced by: Science Popularization China

Author: Denovo Team

Producer: China Science Expo

Many people's little toenails are split, most of the time they are split into two petals, and a few are split into three petals. This kind of split little toenails is professionally called a petal nail, also known as a pián toe. In fact, the proportion of people with petal nails is very high, and it is likely that many people around you have petal nails, but people don't communicate much about it.

Split toenails

(Image source: Wikipedia)

1. Rumors about petal nails

There was a rumor circulating on the Internet that only pure-blooded Han people have split toenails. There are even rumors that one can recognize one's relatives through split toenails. Among them, the rumor about the big locust tree in Hongdong, Shanxi is the most widely circulated:

About 600 years ago, during the Ming Dynasty, Emperor Zhu Yuanzhang ordered the residents of Shanxi to migrate to various parts of the Central Plains (there is also a legend that the Yuan Dynasty decided to migrate some people from Shanxi to fill the population gap in the Central Plains), but the people of Shanxi were unwilling to migrate, and they gathered under the big locust tree in Hongtong. However, at this time, the officers and soldiers surrounded them and forced them to move away. In order to prevent them from escaping, the escorting soldiers cut a knife on their little toenails. Of course, there is another saying that in order to be able to recognize their ancestors in the future, the migrating people cut a knife on their little toenails themselves, so that as long as the little toenails are petal-shaped, they can be sure that they are relatives. Thus, the petal-shaped nails we have today.

In addition to this rumor of the Han people recognizing their relatives, some people also said that the petal-shaped nails were a structure unique to the Mongolians, Manchus and other ethnic groups. These rumors once confused people.

(Source: Weibo)

2. Where are people with petal-like nail trait from?

In fact, toenail splitting caused by trauma cannot be passed on to offspring, and over time, the splitting caused by trauma may heal naturally. Therefore, the above rumors of recognizing relatives by toenails are unscientific.

To understand how petal nails are formed and who has this trait, we have to rely on the power of modern technology.

In 2018, the research team of Tongji University conducted a study on this issue. They selected 2,086 college students from six universities in Shanghai, Anhui Province, and Sichuan Province as research subjects. All subjects were Han Chinese, and their ancestors had no migration records for three generations. The research team used a sampling survey method to gain an in-depth understanding of the incidence of petal nails and the differences between genders in the Han population in 23 provincial administrative regions across the country.

The survey results revealed that the frequency of petal nails in the Han population in Anhui, Fujian, Guangxi, Hebei, Henan, Hubei, Liaoning, Shanxi, Shandong, Shanghai, Zhejiang and other provinces and cities is relatively high, all exceeding 40%; in contrast, the frequency of petal nails in the Han population in Gansu, Guangdong, Heilongjiang and other provinces is relatively low, all around 30%. Among these 23 provincial administrative regions, the frequency of petal nails in the Han population in Liaoning Province is the highest, reaching 45.45%; while the frequency of petal nails in the Han population in Heilongjiang Province is the lowest, only 27.27%. Heilongjiang Province is also the only province where the frequency of petal nails is less than 30%. As a result, the researchers concluded that the distribution trend of petal nails is "the highest occurrence rate in the middle and gradually decreases towards the surrounding areas."

However, the results of a study conducted by researchers at the School of Public Health of Fudan University in 2005 were different. The researchers at that time conducted a survey on the incidence of petal nails in the Han population in seven provinces, namely Shanxi, Shaanxi, Henan, Jiangsu, Zhejiang, Jiangxi and Fujian, and found that the incidence of petal nails was highest in Shanxi and Shaanxi provinces, reaching about 80%.

Why are the results of the 2018 survey so different from those of 2005? In particular, the differences in the incidence of petal nails in the five provinces of Shanxi, Shaanxi, Henan, Jiangsu, and Zhejiang are so obvious. This is because the 2005 study was based on seven relatively closed villages with relatively little communication with the outside world; while the 2018 study was based on the Han population that followed the requirements of native place, and there was genetic exchange between regions, and it was not a completely closed community. In addition, the 2005 study used the incidence of petal nails in one village as data representing the entire province, which has certain limitations.

Types of petal nails, from left to right: normal pinky nail, clear petal nail, fused petal nail, degenerate petal nail, three-petal nail

(Image source: Document 6)

In addition, some people may be curious, are petal nails unique to Chinese people? In a 2016 survey, foreign researchers surveyed subjects from 14 countries or regions including Norway, Germany, and Switzerland, and found that in the 14 countries surveyed, regardless of skin color and race, petal nails were found. So petal nails are not uncommon in foreign countries!

Although the research on the trait of petal nails in my country is not in-depth, the statement that "people with petal nails must be pure Han people" is definitely wrong.

3. What is the cause of the formation of petal-shaped nails?

There are not many genetic studies describing the condition.

In 2005, researchers from Fudan University believed that to determine whether a trait is hereditary, the distribution of the trait in the family should be analyzed to see whether it conforms to the laws of inheritance. So they observed whether the subjects had petal nails, the type of petal nails, gender and age distribution, collected data and sorted out 51 complete family data for pedigree analysis, and found that petal nail inheritance has the following characteristics:

In most families, petal-shaped nails are transmitted continuously and at a high frequency;

There is no gender difference in the presence of petal-shaped nails in the population;

The situation where only the father has petal nails and all his children have petal nails;

If a child has petal-shaped nails, at least one of his parents has petal-shaped nails.

No family was found in which the parents did not have petal nails but the children had petal nails.

In many families, both parents have petal nails but their children do not.

Therefore, the possibility that the trait of petal nails is autosomal recessive, sex-linked, or mitochondrial is ruled out, and it is inferred that petal nails is an autosomal single gene dominant trait.

In 2005, it was considered that petal nails are dominant on autosomes. In the figure above, squares represent males, circles represent females, black fills represent petal nails, white fills represent no petal nails, and half black and half white represent only one foot has petal nails. Taking family 1 as an example, the first generation I:1 (male) has no petal nails, I:2 (female) has petal nails, and their children II:1 (male) has petal nails, II:2 (female) has one foot with petal nails, II:3 (female) and II:4 (female) have petal nails.

(Image source: Document 6)

With the rapid development of gene sequencing technology, a study jointly conducted by researchers from the Chinese Academy of Sciences, the Max Planck Society and Fudan University was published in the Journal of Genetics and Genomics in 2016. The first genome-wide association study of the petal-shaped nail trait in the East Asian Han population revealed a series of characteristics of the petal-shaped toenails.

The sample of this study included 2,980 Han Chinese and 721 Uighurs. The study once again proved that whether or not you have split toenails has nothing to do with whether you are Han Chinese. It was also found that the appearance of split toenails on the left and right feet is not completely symmetrical, and the appearance rate on the right foot is slightly higher than that on the left foot. However, there is no significant difference in this phenomenon between men and women. For adults over 20 years old, age does not cause significant differences in this feature.

Contrary to the previous view that flap nails are inherited through autosomal dominant genes, whole genome scanning has shown that although flap nails are proven to be a heritable trait, no single gene has been found to have a significant effect on the trait of flap nails. It is more likely that it is caused by the combined action of multiple minor effect genes. The effects of minor effect genes on the phenotype are very small, but when they are aggregated together, they may have a significant impact on the phenotype.

This discovery overturned the previously generally accepted view that flap nails conform to Mendel's dominant inheritance law. The researchers believe that the flap nail trait is affected by multiple genes, and the flap nail trait of parents is not necessarily inherited by their offspring - that is, the flap nail trait cannot be used as an obvious genetic feature to identify people of the same ethnic group.

References:

【1】Zhang, Manfei, et al. "Large-scale genome-wide scans do not support petaloid toenail as a Mendelian trait." Journal of genetics and genomics= Yi chuan xue bao 43.12 (2016): 702-704.

【2】Haneke, Eckart. "Double nail of the little toe." Skin appendage disorders 1.4 (2016): 163-167.

【3】Chi, Ching‐Chi, and Shu‐Hui Wang. "Inherited accessory nail of the fifth toe cured by surgical matricectomy." Dermatologic surgery 30.8 (2004): 1177-1179.

【4】Yu Luman, Zhou Yunyun. "A study on the frequency of petal-shaped nails in the Han population of China." Tianjin Science and Technology 45.10(2018):89-91. doi:10.14099/j.cnki.tjkj.2018.10.029.

【5】Zhu Kejing, Jin Yingji, Jin Yuji. The historical origin and genetic research progress of petal nails[J]. Journal of Jilin Medical College, 2017, 38(03): 212-215. DOI: 10.13845/j.cnki.issn1673-2995.2017.03.026.

【6】Hao Weiguo, Bian Jianchao, Zhu Ping. "Types, distribution and inheritance of petal-shaped nails: a survey of a village in Zhouzhi County, Shaanxi Province." Acta Anthropologica Sinica. 04(2005):319-327. doi:10.16359/j.cnki.cn11-1963/q.2005.04.008.

【7】Hao Weiguo, Bian Jianchao. "A study on the types, distribution and inheritance of petal-shaped nails in Hongdong, Shanxi Province." Chinese Journal of Eugenics and Genetics, 01(2005):115-118+2.