Complex sentence: The characteristic of fair skin, red hair, and freckling is associated with high amount of pheomelanin, little amounts of eumelanin, and this is caused by the loss-of-function mutation in the melanocortin 1 receptor (MC1R).
Split into simpler sentences: The characteristic of fair skin, red hair, and freckling is associated with high amount of pheomelanin, little amounts of eumelanin. This phenotype is caused by the loss-of-function mutation in the melanocortin 1 receptor (MC1R).