* Recessive Genes: Hair color is determined by multiple genes, and blonde hair is often a recessive trait. This means a person needs to inherit two copies of the blonde hair gene to express that trait. If both parents carry a recessive gene for blonde hair (even if they have black hair, meaning they have a dominant black hair gene), there's a chance (usually 25%) that their child could inherit both recessive genes and have blonde hair.
* Mutation: A new genetic mutation could occur in the child's genes, resulting in blonde hair. This is less common but possible. The child would then carry this new gene and could potentially pass it on to their offspring.
* Incomplete Penetrance: In some cases, a gene might be present, but its effect isn't fully expressed. This is less likely with hair color but can happen.
* Admixture/Distant Ancestry: If there's distant ancestry with blonde hair (even if not immediately apparent in the family tree), the necessary gene variants could still be present in the family's gene pool and could combine in the child to produce blonde hair.
It's important to note that hair color inheritance can be complex and isn't always straightforward. Genetic testing could provide more specific information if desired, but in most cases, it's due to a combination of recessive genes and genetic inheritance.