Why Do Some People Have Curly Hair While Others Have Straight Hair?

Table of Contents (click to expand)

Whether one’s hair is curly or straight depends on the genes you inherit from your parents. The hair structure and composition also play a role.

There are certain physical attributes of a person that cannot be artificially altered, at least not permanently. This can either be a good or bad thing. For example, height cannot increase after a certain age, skin color cannot change, and straight hair cannot become curly without extensive treatments.

In this article, we will exclusively talk about that last one.

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The Structure Of Hair Follicles

A strand of hair consists of two basic components – the shaft, the black part that appears above the scalp, and the hair follicle that sits in the skin and is not visible.

Hair follicle
Photo credit: en.wikipedia.org

Simply put, a follicle is the organ of the body responsible for producing hair. Quite predictably, the attributes of a hair follicle directly affect the shape and texture of the hair it produces. The shape of the follicle, in turn, is affected by the genes you inherit from your parents.

Also Read: What’s The Science Behind Hair Straighteners/Curlers?

Genes Control Whether You Have Straight Or Curly Hair

Curly hair is an autosomal dominant trait (Source). An autosomal dominant trait is when one parent has a mutated (and dominant) gene while the other does not. People sometimes suffer from an autosomal dominant disorder, which means they have a 50% chance of having a child with a mutant and dominant gene). (Source)

As already mentioned, curly hair is an autosomal dominant trait, which means that one of the parents has straight hair, while the other has curly hair. So you get genes of both types, but the dominant curly hair gene trumps the straight hair gene and expresses itself in the offspring, that is, in yourself. In some cases, if there are differences in the strength and type of genes, you have hair that is somewhere in the middle – a little straight and a little curly.

Also Read: What Are Monogenic And Polygenic Traits?

How Do Genes Affect The Texture Of Hair?

A study conducted in 2009 found that the genes for curly hair are 85-95% heritable, which means that (within the study sample), nine-tenths of the cases of variation in the texture of the hair can be linked back to the DNA of the subjects.

shape of the hair
Image Credit: designua / Shutterstock

It was also found that the nature of the follicles directly affects the curvature of the hair strands. If the follicles are symmetrical, the hair grows straight, while the follicles are asymmetrical; they produce oval-shaped hair that tends to curl as they grow.

Curly Hair And Straight Hair Have Different Compositions

The composition of a strand of hair also influences the texture of the hair, i.e., whether it is straight or curly. In a 2007 study, several human hair samples of different shapes and textures were collected to investigate structural elements related to the hair curl pattern.

Using electron microscopy to compare curly and straight hair fibers, it was observed that the former exhibited an uneven distribution of a specific kind of keratin (keratin is a protein and a primary component of hair), whereas straight hair fibers were symmetrically structured.

There are ways to loosen curly or straighten hair artificially. Some have only a temporary effect (like hair curlers), while some treatments make the changes more permanent, although these have some side effects.

Ultimately, though, it boils down to a simple choice – whether you want to experiment with the hair you were born with or stay with the same hairstyle forever!

References (click to expand)
  1. Medland, S. E., Zhu, G., & Martin, N. G. (2009, October 1). Estimating the Heritability of Hair Curliness in Twins of European Ancestry. Twin Research and Human Genetics. Cambridge University Press (CUP).
  2. Thibaut, S., Barbarat, P., Leroy, F., & Bernard, B. A. (2007, October). Human hair keratin network and curvature. International Journal of Dermatology. Wiley.
  3. Autosomal dominant: MedlinePlus Medical Encyclopedia. MedlinePlus
  4. What causes people to have straight or curly hair?. howitworksdaily.com
About the Author

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spearheads the content and editorial wing of ScienceABC and manages its official Youtube channel. He’s a Harry Potter fan and tries, in vain, to use spells and charms (Accio! [insert object name]) in real life to get things done. He totally gets why JRR Tolkien would create, from scratch, a language spoken by elves, and tries to bring the same passion in everything he does. A big admirer of Richard Feynman and Nikola Tesla, he obsesses over how thoroughly science dictates every aspect of life… in this universe, at least.