Analyse the physiological processes governing human skin colour and its variations. In your answer:

What determines the colour of human skin?

The pigments melanin (which affects how deep the skin tone is) and carotene (which gives an orange/yellow hue). Everyone has the same number of melanocytes; differences in skin tone come down to how active the melanocytes are and how large the melanosomes they produce are.

What are the different pigments and how do they change skin colour?

Melanin — two forms: pheomelanin (red/yellow) and eumelanin (brown/black); determines depth of skin tone. Carotene — orange/yellow hue; fat-soluble, comes from the diet and accumulates in skin layers with high fat content (stratum corneum and subcutaneous layer).

How do melanocytes contribute to skin pigmentation?

Melanocytes are found in the stratum basale. They produce melanin and package it into vesicles called melanosomes. Melanosomes are taken up by basal keratinocytes and fused with lysosomes, releasing the pigment into the keratinocyte.

Why does skin freckle?

Freckles are small dark spots, typically in sun-exposed areas. Melanocytes produce more melanin when the skin is exposed to the sun — the same process as tanning — producing freckles.

Why do some people freckle rather than tan when exposed to the sun?

Your notes state that freckles can appear on anyone but are more common in people with light skin and/or blonde/red hair. They do not explain the underlying mechanism of why some people freckle rather than tan — I'm unable to complete that part without additional information.

What is vitiligo?

A skin condition causing loss of skin pigment; affected areas turn white. Approximately 1% of people have vitiligo, with all races and sexes affected equally.

What are the causes of vitiligo?

Autoimmune disorder — the immune system attacks melanocytes, resulting in no melanin production in those areas. Genetic changes — mutations in various genes involved in melanin production or melanocyte survival can cause cessation of melanin production.