Two types of pigment give hair its color: eumelanin and pheomelanin. Pheomelanin colors hair orange and yellow. All humans have some pheomelanin in their hair. Eumelanin, which has two subtypes of black or brown, determines the darkness of the hair color. A low concentration of brown eumelanin results in blond hair, whereas a higher concentration of brown eumelanin will color the hair brown. High amounts of black eumelanin result in black hair, while low concentrations give gray hair.
Pheomelanin is more chemically stable than black eumelanin, but less chemically stable than brown eumelanin, so it breaks down more slowly when oxidized. This is why bleach gives darker hair a reddish tinge during the artificial coloring process. As the pheomelanin continues to break down, the hair will gradually become orange, then yellow, and finally white.
One phenotype (brown/blonde) has a dominant brown allele and a recessive blond allele. A person with a brown allele will have brown hair; a person with no brown alleles will be blond. This explains why two brown-haired parents can produce a blond-haired child. However, this can only be possible if both parent are heterozygous in hair color- meaning that both of them have one dominant brown hair allele and one recessive allele for blond hair but as dominant always wins the parents both have brown hair. In a punnet square it is possible determine the possibility of a certain trait in an offspring.
The other gene pair is a non-red/red pair, where the non-red allele (which suppresses production of pheomelanin) is dominant and the allele for red hair is recessive. A person with two copies of the red-haired allele will have red hair.
The two-gene model does not account for all possible shades of brown, blond, or red (for example, platinum blond versus dark blond/light brown), nor does it explain why hair color sometimes darkens as a person ages. Several gene pairs control the light versus dark hair color in a cumulative effect. A person's genotype for a multifactorial trait can interact with environment to produce varying phenotypes (see quantitative trait locus).
Blonde hair can have almost any proportion of pheomelanin and eumelanin, but has only small amounts of both. More pheomelanin creates a more golden blonde color, and more eumelanin creates an ash blond. Many children born with blonde hair develop darker hair as they age, with the majority of natural blondes developing a hair color of a dark "gunmetal" hue by the time they reach middle age. Pregnancy hormones hasten this process. Natural blonde hair is rare in adulthood, with some reports stating that only about 2% of the world's population is naturally blonde. Blonde hair is most commonly found in Northern and Western Europeans and their descendants but can be found spread around most of Europe. Blonde hair is rare outside of Europe but can also be found in populations in South America, Israel and Australia. Studies in 2012 showed that naturally blonde hair of Melanesians is caused by a recessive mutation in tyrosinase-related protein 1 (TYRP1).
Pheomelanin is more chemically stable than black eumelanin, but less chemically stable than brown eumelanin, so it breaks down more slowly when oxidized. This is why bleach gives darker hair a reddish tinge during the artificial coloring process. As the pheomelanin continues to break down, the hair will gradually become orange, then yellow, and finally white.
One phenotype (brown/blonde) has a dominant brown allele and a recessive blond allele. A person with a brown allele will have brown hair; a person with no brown alleles will be blond. This explains why two brown-haired parents can produce a blond-haired child. However, this can only be possible if both parent are heterozygous in hair color- meaning that both of them have one dominant brown hair allele and one recessive allele for blond hair but as dominant always wins the parents both have brown hair. In a punnet square it is possible determine the possibility of a certain trait in an offspring.
The other gene pair is a non-red/red pair, where the non-red allele (which suppresses production of pheomelanin) is dominant and the allele for red hair is recessive. A person with two copies of the red-haired allele will have red hair.
The two-gene model does not account for all possible shades of brown, blond, or red (for example, platinum blond versus dark blond/light brown), nor does it explain why hair color sometimes darkens as a person ages. Several gene pairs control the light versus dark hair color in a cumulative effect. A person's genotype for a multifactorial trait can interact with environment to produce varying phenotypes (see quantitative trait locus).
Blonde hair can have almost any proportion of pheomelanin and eumelanin, but has only small amounts of both. More pheomelanin creates a more golden blonde color, and more eumelanin creates an ash blond. Many children born with blonde hair develop darker hair as they age, with the majority of natural blondes developing a hair color of a dark "gunmetal" hue by the time they reach middle age. Pregnancy hormones hasten this process. Natural blonde hair is rare in adulthood, with some reports stating that only about 2% of the world's population is naturally blonde. Blonde hair is most commonly found in Northern and Western Europeans and their descendants but can be found spread around most of Europe. Blonde hair is rare outside of Europe but can also be found in populations in South America, Israel and Australia. Studies in 2012 showed that naturally blonde hair of Melanesians is caused by a recessive mutation in tyrosinase-related protein 1 (TYRP1).
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