Rh (Rhesus) factor is a type of protein found on the outside of red blood cells. It is genetically inherited. An individual that has this protein is Rh-positive, and the one that doesn't is Rh-negative. The Rh-positive gene is dominant, while the Rh-negative gene is recessive. This means that a person must inherit at least one dominant (D) allele from their parents in order to be Rh-positive.
If Dr. Mrs. Brown is Rh-positive, and her father is Rh-negative, her mother must be Rh-positive (DD or Dd), as well. Her father has two recessive alleles (dd), which means that Dr. Mrs. Brown must have one recessive allele. Since she is Rh-positive, she must have one dominant allele. This makes her genotype Dd.
