How do trisomies arise

It can also occur early in development when some cells lose an extra chromosome 21 that was present at conception. The symptoms of someone with mosaic trisomy 21 may vary from those of someone with complete trisomy 21 or translocation trisomy 21, depending on how many cells have the extra chromosome. In this type of chromosomal change, only part of an extra copy of chromosome 21 is in the cells.

The extra part of the chromosome gets "stuck" to another chromosome and gets transmitted into other cells as the cells divide. This type of change causes a small number of Down syndrome cases. There are no distinct cognitive or medical differences between people with translocation trisomy 21 and those with complete trisomy Sometimes, a parent who does not have Down syndrome may carry a translocation in chromosome 21 that can be passed on to children and cause Down syndrome.

In rare cases, only part of chromosome 13 is present in three copies. The physical signs and symptoms in these cases may be different than those found in full trisomy A small percentage of people with trisomy 13 have an extra copy of chromosome 13 in only some of the body's cells. In these people, the condition is called mosaic trisomy The severity of mosaic trisomy 13 depends on the type and number of cells that have the extra chromosome.

The physical features of mosaic trisomy 13 are often milder than those of full trisomy Most cases of trisomy 13 are not inherited and result from random events during the formation of eggs and sperm in healthy parents.

An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 13 in each cell of the body.

Translocation trisomy 13 can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 13 and another chromosome. Colored symbols in the plot represent four different fetal DS brain samples and four astrocyte cell lines obtained from DS brain samples. The results show that the expression of many genes along the entire length of chromosome 21 is increased in DS. The increases observed for APP and SOD1 transcription are fairly typical of many genes, suggesting that their overexpression is not the major cause of DS symptoms.

Importantly, control experiments Figure 6 comparing two normal brain samples showed much lower levels of variation, indicating that the changes in gene expression observed in DS fetal brain samples were not due to random fluctuations associated with the microarray technique.

Overall, the microarray results paint a complex picture of the molecular events that underlie DS. An ongoing challenge for DS research is to sort through the complex set of transcriptional changes associated with DS to identify those genes whose expression is most closely linked to DS phenotypes. To this end, investigators have already constructed mouse models of chromosome 21 overexpression that reproduce some of the symptoms of DS Antonarakis et al.

Researchers hope that these kinds of models will provide useful experimental systems for developing therapeutic interventions for this debilitating condition, which affects millions of people around the world. Antonarakis, S. Chromosome 21 and Down syndrome: From genomics to pathophysiology. Nature Reviews Genetics 5 , — doi FitzPatrick, D. Transcriptional consequences of autosomal trisomy: Primary gene dosage with complex downstream effects.

Trends in Genetics 21 , — doi Hassold, T. Nature Reviews Genetics 2 , — doi Jacobs, P. A case of human intersexuality having a possible XXY sex-determining mechanism. Nature , — doi Mao, M. Global up-regulation of chromosome 21 gene expression in the developing Down syndrome brain. Genomics 81 , — doi Patterson, D. Down syndrome and genetics—A case of linked histories. Nature Reviews Genetics 6 , — doi Chromosome Mapping: Idiograms.

Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities. Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes. Telomeres of Human Chromosomes. Chromosomal Abnormalities: Aneuploidies. Chromosome Abnormalities and Cancer Cytogenetics. Copy Number Variation and Human Disease. Genetic Recombination. Human Chromosome Number. Actions for this page Listen Print. Summary Read the full fact sheet.

On this page. Risk factors for trisomy conditions Trisomy 21 — Down syndrome Trisomy 18 — Edward syndrome Trisomy 13 — Patau syndrome Signs of trisomy conditions during pregnancy Diagnosis of trisomy conditions Genetic counselling and trisomy conditions Where to get help.

Risk factors for trisomy conditions The addition of an extra chromosome usually occurs spontaneously during conception. Trisomy 21 — Down syndrome In Victoria, Down syndrome affects about one in pregnancies. Some of the physical characteristics of Down syndrome may include: slight upward slant of the eyes — nearly all people with Down syndrome have a slight upward slant of the eyes.

Children with Down syndrome tend to grow more slowly and are commonly smaller than other children their age. Adults with Down syndrome are commonly smaller than adults who do not have Down syndrome.

Trisomy 18 — Edward syndrome In Victoria, Edward syndrome affects about one in 1, pregnancies. Some of the characteristics of Edward syndrome may include: physical irregularity of the kidneys, ureters, heart, lungs and diaphragm cleft lip or cleft palate small skull microcephaly malformations of the hands and feet — including missing thumbs, club feet and webbing between the fingers and toes syndactyly neural tube defect, where the spinal cord, meninges and blood vessels protrude through a gap in the vertebrae myelomeningocele malformations of the sex organs.

Survival beyond the neonatal period is uncommon for babies with Edward syndrome. Trisomy 13 — Patau syndrome In Victoria, Patau syndrome affects around one in 3, pregnancies. Some of the characteristics of Patau syndrome may include: small skull microcephaly an abnormal opening in the skull malformations of part of the brain structural defects of the eyes cleft lip or cleft palate additional toes or fingers polydactyly congenital heart disorders, such as ventricular septal defect neural tube defect, where the spinal cord, meninges and blood vessels protrude through a gap in the vertebrae myelomeningocele malformations of the sex organs.

Survival beyond the neonatal period is uncommon for babies with Patau syndrome. Signs of trisomy conditions during pregnancy Sometimes, signs of trisomy conditions may be evident during the pregnancy. Some of these signs may include: too much amniotic fluid surrounding the baby polyhydramnios only one umbilical cord artery a smaller than expected placenta the baby is small for its gestational date the baby is less active than expected congenital defects, including cleft palate or heart irregularities, are picked up during ultrasound scans.

Genetic counselling and trisomy conditions If your child has been diagnosed with a trisomy condition, it may be helpful to speak to a genetic counsellor.