Hair varies greatly among individuals in color, texture, and density. These diverse features are largely shaped by an individual’s genetic makeup.
How Genes Influence Hair
Genes are segments of DNA that contain instructions for various traits, including those related to hair. Each gene exists in different forms called alleles. An individual inherits two alleles for each gene, one from each parent.
The interaction of these alleles determines how a trait is expressed. Some alleles are dominant, meaning they will be expressed if present. Recessive alleles are only expressed when an individual inherits two copies of that specific allele. Many hair traits involve multiple genes and their alleles interacting.
The Genetics of Hair Color
Hair color is primarily determined by the type and amount of melanin, a pigment produced by specialized cells called melanocytes within hair follicles. There are two main types of melanin: eumelanin, which provides black and brown hues, and pheomelanin, responsible for red and yellow tones.
The Melanocortin 1 Receptor (MC1R) gene primarily determines hair color. Variants in the MC1R gene can lead to higher levels of pheomelanin and lower levels of eumelanin, resulting in red hair. This inheritance pattern for red hair is recessive, meaning two copies of specific MC1R variants are needed for its full expression.
Other genes, such as TYR, HERC2, and OCA2, also influence melanin production and distribution, contributing to overall hair color. For example, the TYR gene provides instructions for an enzyme crucial in the initial steps of melanin synthesis. Recent research has identified over 100 genes influencing hair color.
Inheriting Hair Texture and Other Characteristics
Hair texture, such as straight, wavy, or curly, is largely influenced by genetics. The shape of the hair follicle plays a significant role in determining hair texture; curvier follicles tend to produce curlier hair strands. This trait is considered polygenic, meaning multiple genes contribute to its expression.
Specific genes have been identified that influence hair texture. For instance, the EDAR gene, which controls the development of hair follicles, is associated with hair thickness and straightness, particularly in East Asian populations. Variations in the EDAR gene can lead to thicker hair fibers.
Other genes like TCHH and KRTAP (Keratin-Associated Proteins) also contribute to hair texture and the structural properties of hair fibers. The KRTAP genes are essential for forming the rigid and resistant hair shaft.
Genetic Factors in Hair Loss
Genetic predisposition plays a significant role in hair loss, particularly in common conditions like androgenetic alopecia, also known as pattern baldness. This type of hair loss is not caused by a single gene but is a complex trait influenced by multiple genes inherited from both parents. The long-held belief that baldness is inherited solely from the mother’s side is a common misconception, as genes from both maternal and paternal lineages contribute.
Androgenetic alopecia involves an excessive response to androgens, a group of hormones. The androgen receptor (AR) gene, located on the X chromosome, is a well-known genetic factor associated with pattern baldness, especially in males. However, many other genes on various chromosomes also contribute to the risk of hair loss. The expression of these genes can be influenced by factors such as age and hormone levels.