Hair color is determined by the amount and type of melanin pigments produced in the hair follicles. There are two types of melanin: eumelanin which produces brown and black hair, and pheomelanin which produces red hair. The inheritance of hair color follows basic Mendelian genetics. Certain hair colors are dominant over others. Understanding which hair colors are dominant or recessive can help predict the likelihood of a child having a particular hair color based on their parents’ genetics.
The Genetics of Hair Color
There are two genes that determine hair color – the MC1R gene and the OCA2 gene. The MC1R gene codes for a protein involved in producing melanin. Certain variations in this gene reduce the amount of eumelanin produced, causing red hair. The OCA2 gene codes for the P protein which is involved in producing eumelanin. Variants of this gene result in less eumelanin being produced, leading to blond hair.
Each person inherits two copies of each gene, one from each parent. For MC1R, the dominant allele (D) produces eumelanin while the recessive allele (d) results in pheomelanin production. For OCA2, the dominant allele (B) leads to large amounts of eumelanin while the recessive allele (b) causes low eumelanin production.
The combination of alleles inherited for these two genes determines hair color. Some key points about the genetics of hair color include:
- Dark brown/black hair is dominant over lighter shades
- Brown hair is incompletely dominant over blond hair
- Red hair is recessive to all other shades
- The interaction between MC1R and OCA2 alleles produces blended hair colors like auburn
Dominant Hair Colors
The following hair colors are dominant over lighter and red shades:
- Black hair – This is the most dominant hair color. It requires inheritance of dominant alleles for both MC1R (DD) and OCA2 (BB).
- Dark brown hair – This requires DD for MC1R but can have either BB or Bb for OCA2. Dark brown is dominant over lighter browns.
- Light brown hair – This results from having DD for MC1R and bb for OCA2. Light brown is incompletely dominant over blond.
If one parent has a dominant brown or black hair color, the child is extremely likely to also have brown or black hair, regardless of the second parent’s genetics. The dominant dark allele for MC1R is almost always expressed if present.
Intermediate Colors
Some hair colors are produced by a blend of eumelanin and pheomelanin. These include:
- Auburn hair – This reddish brown color results from having Dd for MC1R combined with BB or Bb for OCA2.
- Dark blond hair – Having DD or Dd for MC1R with bb for OCA2 produces this darker yellow/golden shade.
These intermediate hair shades are not truly dominant or recessive. Their expression depends on the specific allele combinations inherited.
Recessive Hair Colors
The following hair colors are only seen when two copies of the recessive alleles are inherited:
- Blond hair – Requires dd for MC1R and bb for OCA2. Blond is recessive to darker brown/black shades.
- Red hair – Caused by inheritance of dd for MC1R. Red hair is recessive to all other shades.
For a child to have blond or red hair, they must inherit two recessive alleles from both parents. If just one dominant allele is inherited, blond/red hair will not be expressed.
Table Summarizing Hair Color Genetics
The inheritance patterns for the main hair colors can be summarized as follows:
| Hair Color | MC1R Alleles | OCA2 Alleles | Inheritance |
|---|---|---|---|
| Black | DD | BB | Dominant |
| Dark brown | DD | BB or Bb | Dominant |
| Light brown | DD | bb | Incompletely dominant |
| Auburn | Dd | BB or Bb | Intermediate |
| Dark blond | DD or Dd | bb | Intermediate |
| Blond | dd | bb | Recessive |
| Red | dd | Any | Recessive |
Predicting Hair Color from Parental Genetics
Using knowledge of dominant and recessive hair colors, we can predict the possible hair shades in children based on their parents’ genetics. Some examples:
- If one parent has black hair (DD, BB) and the other has blond hair (dd, bb), the child could inherit Dd Bb and have dark blond or light brown hair.
- If one parent has auburn hair (Dd, Bb) and the other has red hair (dd, bb), the child could inherit dd Bb and have auburn hair.
- If both parents have black hair (DD, BB), the child will also have DD, BB genotype and black hair.
- If one parent has dark brown hair (DD, Bb) and the other has blond hair (dd, bb), the child could inherit Dd bb and have light brown hair.
To produce a recessive trait like blond or red hair, the child must inherit two recessive alleles from both parents. Otherwise, the dominant alleles for darker pigment will be expressed.
Non-Genetic Factors Affecting Hair Color
While genetics play the primary role, other factors can also influence hair color including:
- Age – Hair often darkens during childhood before lightening in old age as melanin production decreases.
- Sun exposure – Can cause hair to lighten and appear bleached, especially in those with blond or red hair.
- Hair dyes/bleaches – Used cosmetically to alter natural hair color.
- Medical conditions – Some disorders affect melanin production and lead to premature graying.
- Nutrition – Adequate intake of vitamins, minerals, and protein can maximize melanin production.
While these factors may modify hair color, the underlying genetics remain the primary determinants of whether someone has black, blond, brown, or red hair from birth.
Conclusion
In summary, hair color genetics follow classical Mendelian inheritance patterns. Darker shades like black and brown are dominant over lighter shades of blond and red. For blond or red hair to occur, two recessive alleles must be inherited at both the MC1R and OCA2 gene loci. Knowing which alleles are dominant vs. recessive at these two main genes allows predicting the possible hair colors in children based on their parents’ genetics. Hair color mainly depends on the genetics inherited at birth, though other factors like age, sun exposure, and medical conditions can also alter hair color later in life.
