Heredity

Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring.

Heredity in works by Agatha Christie

 * The Harlequin Tea Set - Roland Gilliatt and Timothy Eden are stepbrothers, but look like they are brothers. Roland had inherited his red hair from his father and Timothy has inherited his red hair from his mother. The real Roland is also colourblind like his maternal grandfather.
 * The Lemesurier Inheritance - Hercule Poirot suspects who is the real father of Ronald Lemesurier Jr. is, due to the colour of his hair.