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).
Melanocortin 1 receptor
In the United States, about 25% of the population carries the mutated melanocortin 1 receptor that causes red hair. With one in four people as carriers, the chance of two people having a child with red hair is about 2% (one in 64). The prevalence of red hair varies considerably worldwide. People with freckles and no red hair have an 85% chance of carrying the "MC1R" gene that is connected to red hair. People with no freckles and no red hair have an 18% chance of carrying the "MC1R" gene linked to red hair. Eight genes have been identified in humans that control whether the MC1R gene is turned on and the person has red hair.