Hair care
Scalp hair grows, on average, at a rate of about 1.25 centimeters per month, and shampoos or vitamins have not been shown to noticeably change this rate. Hair growth rate also depends upon what phase in the cycle of hair growth one is actually in; there are three phases. The speed of hair growth varies based upon genetics, gender, age, hormones, and may be reduced by nutrient deficiency (i.e., anorexia, anemia, zinc deficiency) and hormonal fluctuations (i.e., menopause, polycystic ovaries, thyroid disease).
Human hair color
Red hair ranges from light strawberry blond shades to titian, copper, and completely red. It is caused by a variation in the "Mc1r" gene and is recessive. Red hair has the highest amounts of pheomelanin, around 67%, and usually low levels of eumelanin. At 1–2% of the population, it is the least common hair color in the world. It is most prominently found in the British Isles. Scotland has the highest proportion of redheads; 13 percent of the population has red hair and approximately 40 percent carry the recessive redhead gene.