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    the condition in which there are three copies of the a chromosomes type instead of the normal two.

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    Can changes in the number of chromosomes affect health and development?: MedlinePlus Genetics

    A change in the number of chromosomes can cause problems with growth, development, and function of the body's systems. Learn more about these conditions.

    Can changes in the number of chromosomes affect health and development?

    Human cells normally contain 23 pairs of chromosomes, for a total of 46 chromosomes in each cell. A change in the number of chromosomes can cause problems with growth, development, and function of the body's systems. These changes can occur during the formation of reproductive cells (eggs and sperm), in early fetal development, or in any cell after birth. A gain or loss in the number of chromosomes from the normal 46 is called aneuploidy.

    A common form of aneuploidy is trisomy, or the presence of an extra chromosome in cells. "Tri-" is Greek for "three"; people with trisomy have three copies of a particular chromosome in cells instead of the normal two copies. Down syndrome (also known as trisomy 21) is an example of a condition caused by trisomy. People with Down syndrome typically have three copies of chromosome 21 in each cell, for a total of 47 chromosomes per cell.

    Monosomy, or the loss of one chromosome in cells, is another kind of aneuploidy. "Mono-" is Greek for "one"; people with monosomy have one copy of a particular chromosome in cells instead of the normal two copies. Turner syndrome (also known as monosomy X) is a condition caused by monosomy. Women with Turner syndrome usually have only one copy of the X chromosome in every cell, for a total of 45 chromosomes per cell.

    Rarely, some cells end up with complete extra sets of chromosomes. Cells with one additional set of chromosomes, for a total of 69 chromosomes, are called triploid.  Cells with two additional sets of chromosomes, for a total of 92 chromosomes, are called tetraploid.  A condition in which every cell in the body has an extra set of chromosomes is not compatible with life.

    In some cases, a change in the number of chromosomes occurs only in certain cells.  When an individual’s cells differ in their chromosomal makeup, it is known as chromosomal mosaicism.  Chromosomal mosaicism occurs from an error in cell division in cells other than eggs and sperm. Most commonly, some cells end up with one extra or missing chromosome (for a total of 45 or 47 chromosomes per cell), while other cells have the usual 46 chromosomes. Mosaic Turner syndrome is one example of chromosomal mosaicism.  In females with this condition, some cells have 45 chromosomes because they are missing one copy of the X chromosome, while other cells have the usual number of chromosomes.

    Many cancer cells also have changes in their number of chromosomes. These changes are not inherited; they occur in somatic cells (cells other than eggs or sperm) during the formation or progression of a cancerous tumor.

    For more information about chromosomal disorders:

    MedlinePlus provides a medical encyclopedia article about chromosomal mosaicism.

    A discussion of how chromosomal abnormalities happen is provided by the National Human Genome Research Institute.

    The Centre for Genetics Education offers a fact sheet about changes in chromosome number or size.

    Information about chromosomal changes, including changes in the number of chromosomes, is available from EuroGentest.

    The University of Leicester's Virtual Genetics Education Center provides an explanation of numerical chromosome aberrations.

    Your Genome from the Wellcome Genome Campus discusses chromosome disorders, including how changes in the number of chromosomes cause genetic disorders.

    The National Organization for Rare Disorders offers an overview of triploidy.

    Topics in the Variants and Health chapter

    What is a gene variant and how do variants occur?

    How can gene variants affect health and development?

    Do all gene variants affect health and development?

    What kinds of gene variants are possible?

    Can a change in the number of genes affect health and development?

    Can changes in the number of chromosomes affect health and development?

    Can changes in the structure of chromosomes affect health and development?

    Can changes in noncoding DNA affect health and development?

    Can changes in mitochondrial DNA affect health and development?

    What are complex or multifactorial disorders?

    What does it mean to have a genetic predisposition to a disease?

    How are gene variants involved in evolution?

    What information can statistics provide about a genetic condition?

    How are genetic conditions and genes named?

    Other chapters in Help Me Understand Genetics

    The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.

    Source : medlineplus.gov

    Trisomy

    Trisomy

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    Trisomy

    Example of Trisomy 21 detected via qPCR Short Tandem Repeat as

    Specialty Medical genetics

    A trisomy is a type of polysomy in which there are three instances of a particular chromosome, instead of the normal two.[1] A trisomy is a type of aneuploidy (an abnormal number of chromosomes).

    Contents

    1 Description and causes

    1.1 Terminology 2 Human trisomy 3 See also 4 References 5 External links

    Description and causes[edit]

    Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosis creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species. Humans have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 23 chromosomes.

    If the chromosome pairs fail to separate properly during cell division, the egg or sperm may end up with a second copy of one of the chromosomes. ( non-disjunction.) If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.

    Terminology[edit]

    The number of chromosomes in the cell where trisomy occurs is represented as, for example, 2+1 if one chromosome shows trisomy, 2+1+1 if two show trisomy, etc.[2]

    "Full trisomy", also called "primary trisomy",[2] means that an entire extra chromosome has been copied.

    "Partial trisomy" means that there is an extra copy of part of a chromosome.

    "Secondary trisomy" - the extra chromosome has quadruplicated arms (the arms are identical; it is an "isochromosome").[2]

    "Tertiary trisomy" - the extra chromosome is made up of copies of arms from two other chromosomes.[2]

    Trisomies are sometimes characterised as "autosomal trisomies" (trisomies of the non-sex chromosomes) and "sex-chromosome trisomies." Autosomal trisomies are described by referencing the specific chromosome that has an extra copy.[] Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.

    Human trisomy[edit]

    Karyotype of a human with Trisomy 21 (Down syndrome)

    Trisomies can occur with any chromosome, but often result in miscarriage, rather than live birth. For example, Trisomy 16 is the most common trisomy in human pregnancies, occurring in more than 1% of pregnancies; only those pregnancies in which some normal cells occur in addition to the trisomic cells, or mosaic trisomy 16, survive.[3] This condition, however, usually results in spontaneous miscarriage in the first trimester.

    The most common types of autosomal trisomy that survive to birth in humans are:

    Trisomy 21 (Down syndrome)

    Trisomy 18 (Edwards syndrome)

    Trisomy 13 (Patau syndrome)

    Trisomy 9

    Trisomy 8 (Warkany syndrome 2)

    Of these, Trisomy 21 and Trisomy 18 are the most common. In rare cases, a fetus with Trisomy 13 can survive, giving rise to Patau syndrome. Autosomal trisomy can be associated with birth defects, intellectual disability and shortened life expectancy.

    can also occur and include:[4]

    XXX (Triple X syndrome)

    XXY (Klinefelter syndrome)

    XYY

    Compared to trisomy of the autosomal chromosomes, trisomy of the sex chromosomes normally has less severe consequences. Individuals may show few or no symptoms and have a normal life expectancy.[4]

    See also[edit]

    Chromosome abnormalities

    Aneuploidy Karyotype Sexual reproduction Monosomy

    References[edit]

    ^ "CRC - Glossary T". Archived from the original on 2010-06-16. Retrieved 2007-12-23.

    ^ Jump up to:

    Rieger, R.; Michaelis, A.; Green, M.M. (1968). . New York: Springer-Verlag. ISBN 9780387076683.

    ^ Hassold, T; Merrill, M; Adkins, K; Freeman, S; Sherman, S (1995). "Recombination and maternal age-dependent nondisjunction: molecular studies of trisomy 16". . 57 (4): 867–74. PMC 1801507. PMID 7573048.

    ^ Jump up to:

    O'Connor, Clare (2008). "Chromosomal Abnormalities: Aneuploidies". . 1 (1): 172.

    External links[edit]

    Classification D

    ICD-10: Q90-Q92

    Q97-Q98MeSH: D014314SNOMED CT: 78989007

    External resources

    Orphanet: 3376

    hide vte

    Chromosome abnormalities

    Autosomal

    Trisomies/Tetrasomies

    Patau syndrome 13Edwards syndrome 18Down syndrome 21

    Warkany syndrome 2 8Trisomy 9Tetrasomy 9pTrisomy 16Cat eye syndrome/Trisomy 22 22

    Monosomies/deletions

    (1q21.1 copy number variations/1q21.1 deletion syndrome/1q21.1 duplication syndrome/TAR syndrome/1p36 deletion syndrome) 1Wolf–Hirschhorn syndrome 4Cri du chat syndrome/Chromosome 5q deletion syndrome 5Williams syndrome 7Jacobsen syndrome 11Miller–Dieker syndrome/Smith–Magenis syndrome/17q12 microdeletion syndrome 17DiGeorge syndrome 2222q11.2 distal deletion syndrome 2222q13 deletion syndrome 22

    Source : en.wikipedia.org

    Trisomy disorders

    betterhealth.vic.gov.au

    Birth defects

    Trisomy disorders

    Actions for this page

    Trisomy disorders Actions for this page Summary

    Humans have 23 pairs of chromosomes.

    A trisomy is a chromosomal condition characterised by an additional chromosome.

    A person with a trisomy has 47 chromosomes instead of 46.

    Down syndrome, Edward syndrome and Patau syndrome are the most common forms of trisomy.

    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

    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 conditions

    The 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 syndrome

    In 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:

    slight upward slant of the eyes – nearly all people with Down syndrome have a slight upward slant of the eyes. There can also be a small fold of skin on the inside of the eye (called an ‘epicanthic fold’) and small white patches on the edge of the iris of the eye (known as Brushfield spots)

    characteristic facial shape – the face of a person with Down syndrome is often rounded and tends to have a flat profile

    smaller stature – babies with Down syndrome are usually smaller and weigh less at birth than others. 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.

    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 syndrome

    In 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:

    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 3,000 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:

    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

    Source : www.betterhealth.vic.gov.au

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