Genes Controlling Human Hair Color.

 Genes controlling human hair color

PKGhatak, MD

The hair colors of humans are black, brown, blond, grey/white and red. And then there are many shades in each of the colors and in-between colors. It is not usual to see children born to the same couple have different hair colors. This is the magic of genes in the chromosomes.

The MC1R gene is the primary gene for human hair color. The gene MC1R is carried by chromosome 16 on the long arm q at 24.3 location (16q24.3). The deeper layer of the skin contains melanin forming cells melanocytes. MC1R gene in a dominant and active stage produces a good amount of black pigment melanin known as Eumelanin. In MC1R mutation and in the inactive stage produces a yellow-red pigment called Pheomelanin.

In lower mammals, another gene ASIP (agouti suppressor protein) contributes red color. In humans ASIP gene is absent but its role is exhibited in the inactive MC1R gene. Genome wide association study (GWAS) discovered Single Nucleotide Polymorphism (SNP) and its effects on the genes. The SNP in a particular locus modifies the function of the gene in an individual. About 200 SNPs are identified in the European population with blonde to light brown hair color.

In Mendelian inheritance, an individual is either in dominant or recessive mode. In dominant inheritance, there are variations of expression of gene function due to co-dominance. In addition, multiple genes may influence the same characteristic. Varieties of gene interactions are the reasons for so many shades of hair colors and the same parents having different hair colors for babies.

Hair:

Hair has two parts, the root and the shaft. The root of the hair lies deep in the basal layer of the epidermis of the skin. The root of hairs is supplied with branches of blood vessels and nerve fibers. This is the living part of the hair.

The haft that lies above the skin is dead tissue. The center of the shaft is called the medulla and it is surrounded by several layers of densely packed cells forming the cortex. In the cortex the keratinocytes containing the melamine pigment lie. The outer layer is the cuticle, made up of a single layer of cells containing a protein called keratin. Several genes express the texture, length, waves, curls and luster of hair. All these genes, including the MC1R gene, are responsible for the way hair looks.

Melanin.

The melanocytes produce melanin from the amino acid tyrosine. Tyrosine is converted in stages to DOPA then to L-dopaquinone then to eumelanin or phaeomelanin pigment. The long arm of the melanocyte deposits the pigment in the keratinocytes in rows like a string of pearls.

Chemically 5 types of melanin are present but eumelanin and pheomelanin, the two organic melanin are present in human skin, hair, eyes, and adrenal glands, The third one, Neurological, is present in substantia naira of the midbrain, the decreased melamine production is responsible for Parkinson's disease.

Grey / White hair.

As hair grows at the hair root the hair is colorless, and the pigment cells melanocytes surround it. In old age, and in certain diseases and genetic conditions, melanin stops forming. The new hair that emerges looks grey or white. Australia has the most grey haired people in the world due to the effects of bright sunlight on their heads.

Red hair.

Red hair is the least prevalent when the entire world population is considered. Red hair is principally seen in Iris and Scandinavian people and their descendants in other countries.

Blond and different shades of brown hair.

The European population and their descendants carry these genes. As discussed before, multiple genes including NSP polymorphism are responsible for varieties of shades.

Black hair.

African, South Asian, and Chinese people have black hair. Chinese people by a long tradition used to headcovers and they retain black hair color longer in old age than any other race. It is said that stress can change hair to white or grey overnight. That is an exaggeration, however, stress and stress hormones depress melanin formation.

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