Genetic disease: Sickle-cell anemia is a disease that results when a person has two copies of a certain recessive gene. People with one copy of the gene are called carriers. Carriers do not have the disease, but can pass the gene on to their children. A child born to parents who are both carriers has probability 0.25 of having sickle-cell anemia. A medical study samples 18 children in families where both parents are carriers.
a. What is the probability that four or more of the children have sickle-cell anemia?
b. What is the probability that fewer than three of the children have sickle-cell anemia?
c. Would it be unusual if none of the children had sickle-cell anemia?