Autosomal dominant inheritance: Trichorhinophalangeal syndrome type I (TRPS I) is caused by a mutation or defect in the TRPS1 gene. The TRPS1 gene encodes the TRPS I protein and has been identified as a transcription factor with two alternative transcripts, and is expressed in the fetal brain and kidney, and in the adult heart, brain, placenta, lung, liver, skeletal muscle, and pancreas. TRPS I is inherited, or passed down among family members, as a dominant autosomal trait. Individuals receive two copies of most genes, one from the mother and one from the father. For a dominant disorder to appear, only one defective copy of the TRPS1 gene is necessary. If one parent has the disorder, there is a 50% chance that his or her child will have the disorder. If both parents have the disorder, there is a 75% chance that their child will have the disorder.
Autosomal recessive inheritance: Rarely, TRPS I occurs as an autosomal recessive trait. Therefore, a person must inherit two copies of the defective gene, one from each parent. Individuals who inherit only one copy of the defective TRPS1 gene generally have no symptoms and are called carriers because they can pass on the disorder to their children.
If one parent is a carrier, or has only one copy of the defective gene, then each child has a 50% chance of inheriting one defective gene and of also being a carrier. If both parents are carriers, each child has a 25% chance of inheriting two defective genes, a 50% chance of inheriting one defective gene, and a 25% chance of inheriting neither defective gene. Therefore, if both parents are carriers, about one out of four children will have TRPS I.
Sometimes, a person with only one copy of the recessive defective TRPS1 gene will still develop the disorder. This is because the normal copy of the gene cannot compensate for the defective gene, a condition known as haploinsufficiency.
Random occurrence: It is currently unknown whether TRPS I can occur as the result of a spontaneous genetic mutation with no family history of the disease.
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