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Ancestor membership does not transfer phenotype values between groups.","importantHtml":""},"sections":[{"bodyHtml":"<p>Tetrasomy 18p results from the presence of an abnormal extra chromosome, called an isochromosome 18p, in each cell. An isochromosome is a chromosome with two identical arms. Normal chromosomes have one long (q) arm and one short (p) arm, but isochromosomes have either two q arms or two p arms. Isochromosome 18p is a version of chromosome 18 made up of two p arms.</p><p>Cells normally have two copies of each chromosome, one inherited from each parent. In people with tetrasomy 18p, cells have the usual two copies of chromosome 18 plus an isochromosome 18p. As a result, each cell has four copies of the short arm of chromosome 18. (The word &quot;tetrasomy&quot; is derived from &quot;tetra,&quot; the Greek word for &quot;four.&quot;) The extra genetic material from the isochromosome disrupts the normal course of development, causing the characteristic features of this disorder.</p><p class=\"section-source\"><a href=\"https://medlineplus.gov/genetics/condition/tetrasomy-18p/#causes\">MedlinePlus Genetics: Causes</a></p>","key":"forms","title":"What causes Tetrasomy 18p?"},{"bodyHtml":"<p>Babies with tetrasomy 18p often have trouble feeding and may vomit frequently, which makes it difficult for them to gain weight. Some affected infants also have breathing problems and jaundice, which is a yellowing of the skin and the whites of the eyes.</p><p>Changes in muscle tone are commonly seen with tetrasomy 18p. Some affected children have weak muscle tone (hypotonia), while others have increased muscle tone (hypertonia) and stiffness (spasticity). These changes contribute to delayed development of motor skills, including sitting, crawling, and walking.</p><p>Tetrasomy 18p is associated with a distinctive facial appearance that can include unusually shaped and low-set ears, a small mouth, a flat area between the upper lip and the nose (philtrum), and a thin upper lip. Many affected individuals also have a high, arched roof of the mouth (palate), and a few have had a split in the roof of the mouth (cleft palate).</p><p>Additional features of tetrasomy 18p can include seizures, vision problems, recurrent ear infections, mild to moderate hearing loss, constipation and other gastrointestinal problems, abnormal curvature of the spine (scoliosis or kyphosis), a shortage of growth hormone, and birth defects affecting the heart and other organs. Males with tetrasomy 18p may be born with undescended testes (cryptorchidism) or the opening of the urethra on the underside of the penis (hypospadias). Psychiatric conditions, such as attention-deficit/hyperactivity disorder (ADHD) and anxiety, as well as social and behavioral challenges have also been reported in some people with tetrasomy 18p.</p><p class=\"section-source\">Source: <a href=\"https://medlineplus.gov/genetics/condition/tetrasomy-18p\">MedlinePlus Genetics</a></p>","key":"overview","title":"Condition overview"}],"sharedSources":[],"slug":"tetrasomy-18p","sourceDocuments":[{"name":"Tetrasomy 18p","reviewed":"2011-03","sections":{"causes":{"fetchedAt":"2026-10-20T20:25:26.412402+00:00","label":"Causes","pageSha256":"0b1c1e7d9d259536313ac6fbb7b5aa54aad02b2bb8279be55c563fe751768ddf","sourceUrl":"https://medlineplus.gov/genetics/condition/tetrasomy-18p/#causes","status":"available"}},"source_release":"2026-10-01","url":"https://medlineplus.gov/genetics/condition/tetrasomy-18p"}],"sourceNote":"MedlinePlus descriptions and Causes; original HPO annotations.","takeawayHtml":"Cause explanations are from MedlinePlus Genetics. Feature values below retain their original sources and patient groups.","title":"Tetrasomy 18p"},"schemaVersion":1}