My brother must have been deficient in either UDP-glucuronyl transferase (which would have caused the jaundice directly), or UDP-glucose dehydrogenase (which would have caused it indirectly by blocking the production of UDP-glucuronic acid, a substrate for the correct conjugation of bilirubin).
Since bilirubin is a natural product of heme metabolism (recycling of red blood cells, and therefore the hemoglobin they contain), if the production pathways for the enzymes responsible for bilirubin modification for excretion from the liver aren't fully-developed at the time of birth (as the mother's circulation would have been purifying the baby's blood in the placenta before birth), bilirubin will accumulate in the liver.
When it gets high enough in the liver, it will start leaking into the blood serum. Since it's yellowish in color, when it enters the skin capillaries it will color the skin and sclera of the eyesyellow. The reason they probably put my brother under UV lights to treat it is that bilirubin gets broken into manageable pieces by UV light, allowing the liver to send the pieces to the kidneys for excretion until its bilirubin-enzyme handling system is up to snuff.
Cool, huh? Back to studying...