1. Why Straightened Hair Returns: The Hair Follicle's Role
You might have experienced this: you straighten your hair in the morning, but by the time rain falls or a few days pass, the wave is back. It is explained that while straight perms or flat irons can temporarily stretch hair, the original curl emerges once the effect fades. [1] The reason lies in where the hair grows. Hair is created in a sac-like structure called a hair follicle (or hair root) located under the scalp, and it is pushed out from there. This follicle acts as the "mold" for the hair's shape.
According to the Japan Hair Science Association, the cross-section of straight hair is close to a circle, while curly hair often has an elliptical cross-section. The deformation of the cross-section happens because the hair root itself is deformed. The stronger the curl, the greater the degree of deformation is said to be. [1] However, the cross-section shape alone does not determine the strength of the curl. Imagine a tool that pushes out tokoroten (a jelly-like snack). A round hole produces a round cross-section, while a narrow slit produces a narrow one. But hair is made by living cells, so the mechanism is different from pushing out jelly.
You can only straighten the part of the hair that has already grown out. Because the shape of the follicle does not change, the new hair that grows in will have the original curl again. This leads to the question: "If the mold is curved, does the hair also curve?"
2. What Happens Inside a Curved Hair Follicle?
Is it true that the hair follicle is actually curved? In a study that examined small tissue samples from the scalps of African-descent volunteers, curly hair follicles were found to be curved within the dermis. The bulb, the swollen bottom part that produces hair, was also reported to be curved in a shape resembling a golf club. [2] Furthermore, the cells that were multiplying were more numerous on the outer side of the curve (the convex side), creating asymmetry. This study observed curved follicles and curly hair shapes simultaneously, which fits the explanation that "if the follicle is curved, the hair coming out of it will also curve."
However, this research was conducted on the scalps of people of African descent, so it does not necessarily apply to all humans. Also, the cellular mechanism of why follicles curve and how it connects to genes is not yet fully understood. The next question is: "What determines the shape of that mold?"
3. Is Hair Shape Controlled by Just One Gene?
Since hair shape often runs in families, one might imagine there is a single "curly hair gene." However, a genome-wide analysis of Australians of European descent found that hair shape (straight, wavy, or curly) was associated with the trichohyalin gene (TCHH). [3] TCHH works in the inner root sheath of the developing hair follicle. The paper notes that this hair shape is commonly inherited within populations. Still, this gene explained only about 6% of the variation in hair shape among people of European descent. That is 6 out of 100. What causes the rest is not clear from these results alone. [3]
The idea that "one gene determines it" does not seem to fit well for hair shape. So, where were other genes found?
4. Do People in Different Regions Have Different Hair Shape Genes?
In a study of 1,718 people from East Asia (Han, Tibetan, Mongolian, and Lisu groups), the 370A variant of the EDAR gene was associated with straight hair. [5] This variant is linked to hair that is thicker, cylindrical, and straight. In a study of 6,357 people of Latin American descent, hair was scored on a scale (1=straight, 2=wavy, 3=curly, 4=coily). The strongest association with hair shape was found in EDAR, and in addition to TCHH, a new gene called PRSS53 was identified. [6] PRSS53 is also said to work in the inner root sheath during development.
Furthermore, a review article summary states that mutations in keratin genes (such as KRT25, KRT71, and KRT74), as well as CX43, LIPH, and LPAR6, are associated with curliness. [7] Within the scope of the studies reviewed here, TCHH was reported to be associated with hair shape in European-descent groups, while EDAR was associated in East Asian and Latin American groups. However, these are "associations," not guarantees that having a specific gene means you will definitely have straight hair. Within the studies reviewed here, the genes reported to be associated with hair shape differ by population, and how strongly they act or how easily they are detected can differ as well. This view leads to the next question.
5. Did Straight Hair Become More Common Later in Europe, According to Researchers?
In an explanation from the research institute that published the study on Australians of European descent, it is noted that approximately 45% of people of European descent have straight hair, 40% have wavy hair, and 15% have curly hair. [4] The T type of TCHH (rs11803731) is said to increase the frequency of straight hair and is highest in Northern Europe. Researchers explain that the most common variant leads to curly hair, and a shift toward straight hair occurred later within European populations. [4]
However, this is an explanation by researchers regarding European populations. It does not explain the origin of straight hair worldwide. The reason why that variant spread is not clear from this material. Hair shape involves the shape of the follicle, multiple genes, and the history of populations. What is known and what is unknown are still mixed together.
6. How Can You Investigate Hair Shape Yourself?
Things you can do today: Prepare one fallen strand of your own hair. Use a magnifying glass to compare the root side and the tip side. Gently stretch it with your fingers and release it to see how the shape changes. The way it springs back differs from hair to hair. Do not pull out other people's hair to investigate.
Things you can do when you go out: Look for books about hair or genetics at the library.