Final answer:
Gilbert's disease is characterized by a defect in the uptake of bilirubin by liver cells due to lowered UDP-glucuronyl transferase activity. Crigler-Najjar syndrome is caused by a total deficiency of UDP glucuronyl transferase, leading to high levels of serum bilirubin. Both conditions are genetic disorders affecting the liver's ability to process bilirubin.
Step-by-step explanation:
Gilbert's disease is characterized by a defect in the uptake of bilirubin by liver cells due to lowered UDP-glucuronyl transferase activity. On the other hand, Crigler-Najjar syndrome is caused by a total deficiency of UDP glucuronyl transferase, leading to high levels of serum bilirubin. In type-I Crigler-Najjar, the serum bilirubin level exceeds 20 mg/dl. Both conditions are genetic disorders affecting the liver's ability to process bilirubin.