An 11 year old color blin boy is treated with hydroxy-urea for sickle-cell disease.

An 11 year old color blin boy is treated with hydroxy-urea for sickle-cell disease. The physician told him that each of his parents has the sickle-cell trait, but his color blindness was inherited only from his mother.

What cause sickle-cell anemia? HOw does treatment with hydroxy-urea help this condition?
What is sickle-cell and why did the physician state that the boy inherited his sickle-cell disease from both of his parents? why did the physician state that the boy inherited his color blindness from this mother?