Straight-haired parents having a curly-haired child is not uncommon, sometimes surprising people or raising questions about parentage. However, from a genetic perspective, this is entirely normal. Hair type is a hereditary physical characteristic, but it's influenced by multiple genes and can vary across generations. In classical genetics, curly hair is often considered a dominant trait, while straight hair is recessive.

According to a 2016 study published in Nature Communications, only about 6% of hair type variation can be explained by variations in the TCHH (trichohyalin) gene. The rest comes from the interplay of many smaller genes and environmental factors. In many areas like Europe, the prevalence of curly hair ranges from 15% to 30% depending on the region, while in East Asians, straight hair is predominant due to a common variant of the EDAR gene.

Curly hair is a dominant trait, but not more prevalent.

Many assume that a dominant gene is always more frequent in a population. In reality, dominant only means it's expressed (phenotype) if present in the genome, not that it's more common. For example, Huntington's disease is caused by a dominant gene, but it's rare due to its negative health effects, leading to its gradual elimination by natural selection.

In the case of hair, the gene for curly hair may be dominant over straight hair, but its expression depends on factors such as ethnicity, evolutionary history, and ancestral distribution. Therefore, even if both parents have straight hair, they can still have a curly-haired child if they both carry the recessive gene for curly hair.

Polygenic inheritance and random combinations.

Hair type isn't determined by a single gene but is a polygenic trait. This means multiple genes contribute to hair curl, thickness, and texture. When both parents have straight hair but carry different gene variants related to curliness, their child can inherit a combination that results in curly hair.

Specifically, in a genetic model called "incomplete dominance," if an individual has one gene for curly hair and one for straight hair, the hair type might be intermediate, like wavy. If the child receives two copies of the curly hair gene, their hair can be noticeably curly. This is why some children have strikingly curly hair even with straight-haired parents.

Furthermore, due to ancestral diversity and gene mutations over generations, variants not expressed in previous generations can unexpectedly appear in offspring. For instance, if a grandparent had curly hair, the gene could be passed down silently for generations before manifesting in a child.

Environmental and epigenetic influences.

Besides genetics, hair type is influenced by physiological and environmental factors. Hormones, hormonal changes, diet, and even weather conditions can affect hair texture. Some people are born with straight hair that becomes curly during puberty or after pregnancy due to shifts in estrogen and androgen levels. A study at the University of Bradford (UK) showed that hormonal changes can affect hair follicle shape, directly impacting hair curl.

Additionally, physical factors like hair care practices, heat styling, and chemical treatments also contribute to changes in curl, making it difficult to distinguish between genetic and habit-driven changes.

Curly hair is often considered a dominant trait, while straight hair is recessive. Photo: Pexel

Genetic diversity among human populations.

Ethnicity plays a significant role in hair type inheritance. People of Asian descent, particularly East Asians, have a high prevalence of straight hair due to the common EDAR V370A gene variant, which results in round hair follicles and straight hair growth. Meanwhile, people of African descent have a high prevalence of curly hair due to genetic characteristics that lead to flatter hair follicles. People of European descent often have a mix, leading to a range of hair types from straight to wavy and curly.

This diversity explains why curly hair in children of Asian couples is more surprising than in other populations. However, with increasing migration and interracial marriages, the likelihood of expressing diverse genetic traits also rises.

Understanding to avoid misconceptions.

A child having different hair from their parents sometimes leads to unwarranted suspicions, especially in East Asian societies that value physical resemblance within families. However, modern genetics provides ample evidence that hair type is a complex trait influenced by multiple genes and other factors; it can't be judged by appearance alone.

With increasingly accurate genetic analysis tools, explaining hereditary traits like hair type, eye color, or blood type has become clearer. However, it's crucial to effectively communicate basic genetic knowledge to avoid prejudice and misconceptions in the community.

Straight-haired parents having a curly-haired child isn't an exception but a natural outcome of gene combinations according to Mendelian and modern genetics. By understanding this mechanism, society can reduce emotional reactions and approach biological phenomena with a more scientific and open perspective.

Thuc Linh (According to The Tech, GB Health Watch, Healthline)