Final answer:
The assumption that blue hair is dyed is justified as natural human hair color is determined by melanin, with no natural shades of blue or green. DNA determines traits and does not naturally code for pigments resulting in these colors. While undiscovered genetic expressions are possible, naturally occurring blue or green human hair is extremely unlikely.
Step-by-step explanation:
Understanding Human Hair Color Assuming that people with blue hair have dyed it is generally justified because human hair color is based on the pigmentation of hair follicles due to melanin, specifically eumelanin and pheomelanin. The range of natural human hair colors does not include blue. The theoretical possibility of radiation affecting hair color during embryonic development is severely limited by our understanding of genetics and molecular biology; repercussions would likely be detrimental rather than just cosmetic. The DNA molecule, which largely determines our traits, has not been known to naturally code for pigments that would result in blue or green human hair.Genetic linkage is a concept that somewhat explains why certain characteristics, like hair color and eye color, often occur together.
Melanin production, and therefore hair pigment, decreases with aging, leading to gray or white hair. Anthropic variations in skin and hair relate more to adaptations to different environmental conditions over history than to the singular effect of a radiation event on embryogenesis.In conclusion, while it's not impossible for there to be undiscovered alleles of genes affecting hair color, the likelihood of naturally occurring blue or green human hair due to unique circumstances or mutations is extremely low. The NCBI's repository on alleles, such as those affecting the MC1R gene, is quite comprehensive for known variations, and while not exhaustively predictive of all possible genetic expressions, represents a wide array of researched human phenotypes.