User:Gcrossan/biochem

Biochemistry and genetics

Biochemistry

Pigment of cells and hair is governed by the Melanocyte-stimulating hormone (MSH). This binds with the melanocortin-1 receptor (MC1R) on melanocyte cell membranes. Through internal signalling, pigments are then synthesised, these being the red pigment pheomelanin and the black eumelanin.

In people with red hair, there is a mutation in the gene coding for MC1R (found on chromosome 16), thus affecting the levels of eumelanin in the cell. The less eumelanin the cell contain, the fairer the colour of the skin and, since the cell has high levels of pheomelanin, also results in red hair.

People with red hair are more susepticle to cellular damage from UV light.

Genetics

Valverde et al. (1995) used PCR to sequence the defective gene and found 9 variants:

• Valine at position 60 was changed to a Lysine
• Alanine at position 64 was changed to a Serine
• An extra Alanine inserted at position 86
• Valine at position 92 was changed to a Methionine
• Arginine at position 142 was changed to a Hisidine.
• Arginine at position 151 was changed to a Cysteine.
• Arginine at position 160 was changed to a Tryptophan.
• Aspartic Acid at position 294 was changed to a Histidine.
• An extra Cysteine inserted at position 537

???There is some indication that the uncommon pheomelanin/eumelanin ratios found in redheads may be correlated with some corresponding variations in the abundance of other hormones and neuropeptides, including epinephrine (adrenaline), dopamine, and oxytocin. Wolves which are bred to be tame have been found to acquire a progressively paler coat of fur as they become tamer and tamer through successive generations. The speculation is that the cell biology which produces epinephrine (adrenaline) needed for the high-energy fight-or-flight response is linked to the cell biology that governs the relative production of pheomelanin and eumelanin. This finding might explain why redheads are often characterized as having a distinct temperament compared to the rest of the population. ????

. The MC1R recessive gene, which gives people red hair and fair skin, is also associated with freckles, though it is not uncommon to see a redhead without freckles. Eighty percent of redheads have a MC1R gene variant,

the prevalence of these alleles is highest in Scotland and Ireland. The alleles that code for red hair occur close to the alleles that impact skin color, so it seems that the phenotypic expression for lighter skin and red hair are interrelated. The genetics of red hair are now being uncovered, together with connections between red hair and melanoma, skin disorders in general, and different reactions to anaesthesia. There is evidence of genetic linkage of eye color with other hair colors, such as brown hair, although MC1R is not linked to eye color. The inheritance of red hair is close to what geneticists describe as an autosomal recessive mode of inheritance. This means that the parents of red haired children may carry the gene for red hair but not have red hair themselves.

There is also evidence that red hair may be an example of incomplete dominance. This means that when only one copy of the red hair allele is present, red hair may blend with the other hair color, resulting in different types of red hair including strawberry blond (red-blonde) and auburn (red-brown).

The alleles Arg151Cys, Arg160Trp, Asp294His, and Arg142His on MC1R are shown to be recessives for the red hair phenotype.

Also possibly related to red hair is the gene HCL2 (aka RHC, RHA) on chromosome 4.

Evolution

Red hair is the rarest type of natural hair color in humans. The pale skin associated with red hair may be of advantage in far-northern climates where sunlight is scarce. Studies by Bodmer and Cavalli-Sforza (1976) hypothesized that lighter skin pigmentation prevents rickets in colder latitudes by encouraging higher levels of Vitamin D production and also allows the individual to retain heat better than someone with darker skin. Rees (2004) suggested that the vividness and rarity of red hair may lead to it becoming desirable in a partner and therefore it could become more common through sexual selection.

Harding et al (2000) proposed that red hair was not the result of positive selection but rather occurs due to a lack of negative selection. In Africa, for example, red hair is selected against because high levels of sun would be harmful towards fair skin. However, in Northern Europe this does not happen and so redheads come about through genetic drift.

Estimates on the original occurrence of the currently active gene for red hair vary from 20,000 to 100,000 years ago.