Although Robert and Cora Crawley both have blue eyes, their daughter, Mary, has brown eyes. This was thought biologically impossible in the Davenport Model (1907), although not very common, two blue-eyed parents can produce children with brown eyes. Eye color is a complex trait that depends on the state of several interacting genes. The gene that usually decides the issue (blue eyes or brown eyes) is the OCA2 gene on chromosome 15. But it comes in different strengths. A person with a weak form of the OCA2 gene will have blue eyes. Likewise a person with a strong form will have brown eyes, but individuals also have other eye-color genes that each has a say in the final eye-color outcome. For example, if one of these lesser genes is strong, it can make the weak form (blue) of OCA2 work much more effectively - almost like the strong form (brown). Then the eye color may be brown. In fact, the resulting color can be any shade of brown, hazel, green or blue depending on the interactions.