Regular haircut
Human scalp hair grows on average about one eighth inch per week or one half inch per month. Most clipper guards are numbered in eighths of an inch. The number of the guard denotes the number of weeks of hair growth left on the scalp when a clipper with a certain numbered guard is guided over the head with the guard in contact with the scalp. A #1 guard leaves 1/8 inch (3 mm), one week's growth of hair, on the scalp; a #2 guard leaves 2/8" (6.3 mm), two weeks' hair growth, on the scalp; a #3 guard leaves 3/8" (9 mm), three weeks' hair growth on the scalp; and so on.
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.