Hypo as said above is hypomelanistic. Essentially a lack of dark pigment. Thus why it tends to glow and be lighter. Ghost was deemed to create a metaphor if you will to help visualize this definition
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Now hypomelanistic and amelanistic are similar in definition but you have to know what happens to the melanin. Amelanistic is albino and there is melanin, but it was not produced correctly by certain enzymes in the developmental stages. It is genetic but it's basically an abnormality.