Here's why:
* Genetics: Hair color is primarily determined by the amount and type of melanin (a pigment) in the hair. The MC1R gene plays a significant role in producing melanin. Different alleles (versions of the gene) of MC1R can lead to different amounts and types of melanin.
* Dominance: The allele for brown hair typically leads to the production of more melanin than the allele for blond hair. Because brown hair requires more melanin than blond hair, the alleles for brown hair are considered dominant. The allele for blond hair produces less melanin and is considered recessive.
* Inheritance: For a person to have blond hair, they generally need to inherit two copies of the recessive blond hair allele (one from each parent). If a person inherits at least one copy of the dominant brown hair allele, they will typically have brown hair.
Important Considerations:
* Not a Simple Mendelian Trait: While often taught as a basic example of Mendelian inheritance, hair color is actually more complex than just one gene. Other genes also contribute to hair color, leading to a range of shades and variations.
* Other Colors: This dominance relationship primarily applies to brown versus blond. Red hair, for example, involves a different set of genes and interactions.
* Variations: Even with a dominant brown hair allele, the exact shade of brown can vary depending on other genetic factors and environmental influences.
In summary, while not a perfectly simple example, brown hair is generally considered dominant to blond hair in terms of genetic inheritance.