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

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 in a truly permanent way. This can either be a good or bad thing, depending on the person you’re talking to. For instance, one’s height cannot increase (visibly) after a certain age, skin color cannot change and straight hair can’t become curly (without extensive treatments). In this article, we will exclusively talk about that last one.

The structure of Hair follicles

A strand of hair has two basic components – the shaft (the black part that appears above the scalp) and the hair follicle (which resides in the skin and is not visible).

Hair follicle

Photo credit: en.wikipedia.org

In simple words, you can say that a follicle is the organ of the body responsible for producing hair. Quite predictably, the attributes of a hair follicle directly impact the shape and texture of the hair that it produces. The shape of the follicle, in turn, is affected by the genes the you inherit from your parents.

Genes control whether you have straight or curly hair

Curly hair is an autosomal dominant trait (Source). An autosomal dominant trait is one wherein one of the parents (father or mother) has a mutated (and dominant) gene, while the other doesn’t. People sometimes suffer from an autosomal dominant disorder, which means that they run a 50% chance of having a child with a mutated (and dominant gene). (Source)

As mentioned earlier, 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, i.e., yourself. In some cases, when there are differences in the strength and type of genes, you end up having hair that is somewhere in the middle – a little straight and a little curly.

How do genes affect the texture of hair?

In a study conducted in 2009, it was found that the genes of curly hair have a heritability of 85-95%, 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 follicles directly impacts the curvature of hair strands. If the follicles are symmetrical, hair grows to be straight, whereas if the follicles are asymmetrical, they produce oval-shaped hair that tends to curl as it grows.

Curly hair and straight hair have different compositions

The composition of a hair strand also impacts the texture of hair, i.e., whether it’s straight or curly. In a study conducted in 2007, multiple human hair samples of different shapes and textures were collected to investigate structural elements that might be related to hair curl pattern.

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

There are ways through which you can curl/straighten your hair artificially. Some only have a temporary effect (like hair curlers), while some treatments make the changes more permanent, although these do have some side effects.

, Why Do Some People Have Curly Hair While Others Have Straight Hair?, Science ABC, Science ABC

However, in the end, it all comes down to one simple choice – whether you want to experiment with the hair you’re born with or stick with the same hairdo for all eternity!

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About the Author

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

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