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Genes are the blueprint for our bodies. Almost every cell in the body has a copy of the blueprint, stored inside a sac called the nucleus. Genes are beaded along chromosomes, which are tightly bundled strands of the chemical substance deoxyribonucleic acid (DNA). Humans usually have 23 pairs of chromosomes, with two sex chromosomes that determine sex and 44 chromosomes that direct other factors, such as growth and function. A chromosome condition is caused by an alteration in the number or genetic structure of chromosomes. Trisomy (‘three bodies’) means the affected person has three copies of one of the chromosomes instead of two. This means they have 47 chromosomes instead of 46. Down syndrome, Edward syndrome and Patau syndrome are the most common forms of trisomy. Children affected by trisomy usually have a range of birth anomalies, including delayed development and intellectual disabilities. Risk factors for trisomy conditionsThe addition of an extra chromosome usually occurs spontaneously during conception. The cause of this is unknown and prevention is not possible. The most important risk factor for trisomy conditions is maternal age. Women in their late 30s and 40s have a higher chance of trisomy conditions occurring. Trisomy 21 – Down syndromeIn Victoria, Down syndrome affects about one in 300 pregnancies. Down syndrome is also known as Trisomy 21, because the person has three copies of chromosome 21 instead of two. There are three types of Down syndrome. The most common is Standard Trisomy 21, in which the father’s sperm or the mother’s egg cell contains the extra chromosome. In Mosaic Down syndrome, the extra chromosome spontaneously appears as the embryo develops. Translocation Down syndrome, which accounts for approximately five per cent of cases, is inheritable. Some of the physical characteristics of Down syndrome may include:
All people with Down syndrome will experience some delay in their development and some level of learning disability. Learn more about Down syndrome. Trisomy 18 – Edward syndromeIn Victoria, Edward syndrome affects about one in 1,100 pregnancies. Edward syndrome is also known as Trisomy 18, because the person has three copies of chromosome 18 instead of two. Some of the characteristics of Edward syndrome may include:
Survival beyond the neonatal period is uncommon for babies with Edward syndrome. Trisomy 13 – Patau syndromeIn Victoria, Patau syndrome affects around one in 3,000 pregnancies. Patau syndrome is also known as Trisomy 13, because the person has three copies of chromosome 13 instead of two. Some of the characteristics of Patau syndrome may include:
Survival beyond the neonatal period is uncommon for babies with Patau syndrome. Signs of trisomy conditions during pregnancySometimes, signs of trisomy conditions may be evident during the pregnancy. Some of these signs may include:
Diagnosis of trisomy conditionsPrenatal tests that can help detect trisomy disorders include: Genetic counselling and trisomy conditionsIf your child has been diagnosed with a trisomy condition, it may be helpful to speak to a genetic counsellor. Genetic counsellors are health professionals qualified in both counselling and genetics. As well as providing emotional support, they can help you to understand your child’s condition, what causes it, and what a diagnosis means for your child's health and development. Genetic counsellors are trained to provide information and support that is sensitive to your family circumstances, culture and beliefs. The Genetic Support Network of (GSNV) is connected with a wide range of support groups throughout Victoria and Australia and can connect you with other individuals and families affected by trisomy conditions. Where to get helpThis page has been produced in consultation with and approved by: This page has been produced in consultation with and approved by: This page has been produced in consultation with and approved by:
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