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The dark pigment in skin and hair is called melanin which is made in melanocytes; these are activated by the melanocyte-stimulating
hormone (MSH).There are receptors for MSH on the surface of melanocyte cells. People with more receptors have darker skin and hair.
UV light increases the amount of MSH and MSH receptors, making the melanocytes more active and causing the skin to darken.
Hair lightens due to the destruction of melanin by UV light. This can cause seasonal colour change e.g. in the Arctic fox.
To make melanin, animals use an enzyme called tryosinase. This catalyses the reaction when the amino acid tyrosine turns into melanin.
Some animals can have mutant alleles for tryosinase; the enzyme is made but is unstable and inactivated at normal body temperature.
Extremities in animals such as the Himalayan rabbit are slightly cooler and so the enzyme remains active and melanin is made.
Monoamine oxidase A is an enzyme that catalyses the breakdown of a neurotransmitter in the brain involved in the regulation of behaviour,
including the response to stress.
It was found that some individuals have a rare mutation in the MAOA gene and produce no enzyme; they exhibit aggressive and
sometimes violent behaviour.
However there are several environmental factors that can influence aggression e.g. childhood maltreatment.