“My daughter’s hair is straight, but my hair and her father’s hair are both curly. Isn’t curly hair dominant?”
According to “Ask a Geneticist” on The Tech Museum of Innovation’s website and “What causes people to have straight or curly hair?” by Robert Jones on www.howitworksdaily.com, curly hair is a dominant trait. However, if both parents actually have wavy hair, it’s possible for them to have a straight-haired daughter.
Remember doing Punnett squares in biology class? CC represents the curly hair gene, and ss represents the straight hair gene. If both parents have curly hair, they can each only contribute a C, so their children will have curly hair too. The same goes for two straight-haired parents – they can each only contribute an s, so their children will have straight hair. What about a child with one straight-haired parent and one curly-haired parent? Because one parent contributed a C and the other contributed an s, the child will have both genes and their hair will be wavy. (Although curly hair is technically dominant, hair type is an example of incomplete dominance, so the curly hair doesn’t cancel out the straight hair entirely.)
Wavy hair, then, is represented by Cs. A wavy-haired parent can either contribute a C or an s, so two wavy-haired parents have a fifty percent chance of having a wavy-haired child, a twenty-five percent chance of having a curly-haired child, and a twenty-five percent chance of having a straight-haired child. It’s the shape of the hair follicles that determine the shape and texture of the hair: rounder follicles will produce straight hair, while more oval follicles will produce curlier hair.
To make things even more interesting, according to Jessica Goldstein’s article for NPR “A Hair Mystery: Curly Hair Gone Straight," some people report that their hair’s been known to change shape and texture on its own as they age. No one’s quite certain exactly why it happens, though changes in hormones and body chemistry probably factor into it.
For more information on the secrets of genetics, Sam Kean’s The Violinist’s Thumb:and Other Lost Tales of Love, War, and Genius, as Written by Our Genetic Code is available at the Newton Falls Public Library.