"MC1R only explains 73% of the SNP heritability for red hair in UK Biobank, and in fact most individuals with two MC1R variants have blonde or light brown hair."
If we assume that a dark hair strand has 10,000 "molecules" of total melanin, then by the ratios above, 9900 “molecules” (99%) of these would be eumelanin and 100 "molecules" (1%) would be pheomelanin. In comparison, a blond hair strand of equal size may have only 100 "molecules" of total melanin. Of these, 95 "molecules" are eumelanin (95%) and 5 "molecules" are pheomelanin (5%).
When both dark and blond hair types are bleached, most of the accessible eumelanin in the two types of hair would be broken down and discolored, and only very little of the pheomelanin would do so. The reason why pheomelanin is harder to bleach is because it has sulfur linkages (or ties) that make the molecule more tightly packed and inaccessible to the bleach.The end result is that dark hair lifts to a brassy undertone because of the significant amount of pheomelanin left behind (let’s say 90 molecules), while blonde hair reaches the palest yellow stage because only very few pheomelanins were there to start with, and they get even less after the bleach (Let’s say 3 or 4 molecules).