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Down Syndrome

Fact Sheet

What is Down Syndrome?

Down Syndrome is a genetic condition resulting from an extra 21st chromosome which often causes delays in physical and intellectual development. It occurs in approximately 1 in every 800 live births. Individuals with Down Syndrome have 47 chromosomes instead of the usual 46.

Three genetic variations can cause Down Syndrome. Approximately 92% of the time, Down Syndrome is caused by the presence of an extra Chromosome 21 in all cells of the individual. In such cases, the extra chromosome originates in the development of either the egg or the sperm. Consequently, when the egg and sperm unite to form the fertilized egg, three, rather than two, Chromosome 21’s are present. As the embryo develops, the extra chromosome is repeated in every cell. This condition, in which three copies of Chromosome 21 are present in all cells of the individual, is called Trisomy 21.

In approximately 2-4% of cases, Down Syndrome is due to Mosaicism. This situation is similar to Trisomy 21 but, in this instance, the extra Chromosome 21 is present in some, but not all, cells of the individual. For example, the fertilized egg may have the right number of chromosomes, but, due to an error in chromosome division early in embryonic development, some cells acquire an extra Chromosome 21. Thus, an individual with Down Syndrome due to Mosaic Trisomy 21 will typically have 46 chromosomes in some cells, but will have 47 chromosomes (including an extra Chromosome 21) in others. In this situation, the range of the physical problems may vary, depending on the proportion of cells that carry the additional Chromosome 21.

The third genetic variation is called Translocation. This refers to the type of Down Syndrome that is caused by rearranged chromosome material. There are three 21 chromosomes, similar to Trisomy 21 but in this case, one of the 21st chromosomes has attached to another chromosome instead of being separate. Only 3-4% of babies born with Down Syndrome will have Translocation.

How is it manifested?

  • muscle hypotonia (low muscle tone)
  • flat facial profile (a somewhat depressed nasal bridge and a small nose)
  • oblique palpebral fissures (an upward slant to the eyes)
  • dysplastic ear (an abnormal shape of the ear)
  • a single deep crease across the center of the palm
  • hyperflexibility (an excessive ability to extend the joints)
  • epicanthal folds (small skin folds on the inner corner of the eyes)
  • excessive space between large and second toe
  • enlargement of tongue in relationship to size of mouth

Who is affected?

It is estimated that 1 in every 800-1,000 children born in Canada are diagnosed with Down Syndrome. It is the most frequently-occurring chromosomal abnormality.

How is it diagnosed or detected?

Down Syndrome is usually identified at birth or shortly thereafter. Initially, the diagnosis is based on physical characteristics that are commonly seen in babies with Down Syndrome. These include low muscle tone, a single crease across the palm of the hand, a slightly flattened facial profile, and an upward slant to the eyes. The diagnosis must be confirmed by a chromosome study (karyotype). A karyotype provides a visual display of the chromosomes grouped by their size, number, and shape. Chromosomes may be studied by examining blood or tissue cells.

There are also two types of prenatal tests used to detect Down Syndrome in a fetus: screening and diagnostic tests. The screening test estimates the risk that a fetus has Down Syndrome while a diagnostic test determines whether or not the fetus has a condition with an accuracy of 99%. However, diagnostic tests are more invasive since it is tested in utero. Contact your health care provider for more information.

Additional Resources:

Down Syndrome Association of
Down Syndrome Association of Toronto is a non-profit organization providing support and information to parents of children with Down Syndrome, students and teachers.

Canadian Down Syndrome Society (CDSS)
The CDSS is a resource linking parents and professionals through advocacy, education and providing information.

National Down Syndrome
It is the purpose of the National Down Syndrome Congress to create a national climate in which all people will recognize and embrace the value and dignity of people with Down Syndrome

Books and Literature:
Parentbooks offers the most comprehensive selection of resources available anywhere – from planning a family, to everyday parenting issues, to special needs of all kinds. It also has a selection of resources for caregivers, counselors, therapists, educators, and clinicians.

Babies With Down Syndrome. A New Parents’ Guide
by Karen Stray-Gunderson Edited by Susan Skallerup

Classroom Language Skills in Children with Down Syndrome
By Libby Kumin

Discovery: Pathways to Better Speech for Children with Down Syndrome (DVD)
by Libby Kumin, Dr. Sally Shott, Professor Sue Buckley, and Dr. Seigfried Puschel

Down Syndrome: the First 18 Months (DVD)
By Will Schermerhorn

The Down Syndrome Nutrition Handbook: A Guide to Promoting Healthy Lifestyles
By Joan E. Guthrie Medlen

The Down’s Syndrome Handbook: A Practical Guide for Parents and Carers, Revised Edition
By Richard Newton

Early Communication Skills for Children with Down Syndrome: A Guide for Parents and Professionals
By Libby Kumin

Early Education for Children with Down Syndrome (Time to Begin)
By Valentine Dmitriev

The content contained in this document is for general information purposes. It is not the intention to diagnose or treat a child.

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