Blue diaper syndrome is a rare disorder inherited as an autosomal recessive trait although X-linked recessive inheritance has not been completely ruled out. Genetic diseases are determined by two genes, one received from the father and one from the mother.
Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%.
X-linked recessive genetic disorders are conditions caused by an abnormal gene on the X chromosome. Females have two X chromosomes but one of the X chromosomes is “turned off” and all of the genes on that chromosome are inactivated. Females who have a disease gene present on one of their X chromosomes are carriers for that disorder. Carrier females usually do not display symptoms of the disorder because it is usually the X chromosome with the abnormal gene that is “turned off”. Males have one X chromosome and if they inherit an X chromosome that contains a disease gene, they will develop the disease. Males with X-linked disorders pass the disease gene to all of their daughters, who will be carriers. Males can not pass an X-linked gene to their sons because males always pass their Y chromosome instead of their X chromosome to male offspring. Female carriers of an X-linked disorder have a 25% chance with each pregnancy to have a carrier daughter like themselves, a 25% chance to have a non-carrier daughter, a 25% to have a son affected with the disease, and a 25% chance to have an unaffected son.
Symptoms, such as the blue urine stains on diapers, develop due to the intestinal breakdown of excessive amounts of tryptophan and the accumulation of indican and related compounds (e.g., indigotin) in the urine (indicanuria). When tryptophan is broken down by intestinal bacteria, it is converted into an organic compound called indole. Indole is absorbed and broken down into another organic compound called indican. When exposed to air, indican converts into indigo blue dye giving the urine a distinctive blue color. Although the exact nature of the biochemical defect remains uncertain, it is believed to be related to a defect in the intestinal absorption and transport of tryptophan.