Robert and Cora Crawley both have blue eyes, but their daughters Mary and Edith have brown eyes. While it's not very common, two blue-eyed parents can produce brown-eyed children. Eye color is a complex trait that depends on the state of several interacting genes. The OCA2 gene on chromosome 15, which usually determines eye color, comes in different strengths. A person with a weak form of the gene will have blue eyes, and a person with a strong form will have brown eyes. Individuals also have other eye-color genes. For example, if one of these lesser genes is strong, it can make the weak form (blue) of OCA2 work much more effectively. Depending on the interactions of other genes, the resulting eye color can be any shade of brown, hazel, green or blue.