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Questions and Answers
What proportion of individuals with Down Syndrome have the trisomy 21 due to non-disjunction?
Maternal age has a strong influence on the incidence of Down Syndrome. What is the incidence in women over 45 years?
What percentage of individuals with Down Syndrome are mosaics?
What is the cause of Down Syndrome in approximately 4% of cases?
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Which of the following regarding sex chromosome disorders is true?
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In cases of Down Syndrome, where does the extra chromosome usually originate from?
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Study Notes
Trisomy 21 (Down Syndrome)
- Trisomy 21 is the most common chromosomal disorder.
- 95% of affected persons have trisomy 21, usually due to non-disjunction.
- In 95% of cases, the extra chromosome is maternal in origin.
- Parents of affected individuals are typically normal.
- Maternal age strongly influences incidence, with a rate of 1 in 1550 live births in women under 20, and 1 in 25 live births in women over 45.
- Paternal age has no effect on incidence.
Trisomy 21 Variations
- In 4% of cases, the extra chromosomal material is present as a translocation of the long arm of chromosome 21 to chromosome 22 or 14, and is often familial.
- Approximately 1% of trisomy 21 cases are mosaics, with a mixture of 47 and 46 chromosome cells, resulting in variable and milder symptoms.
Cytogenetic Disorders Involving Sex Chromosomes
- Abnormal karyotypes involving sex chromosomes, ranging from 45X to 49XXXXY, are compatible with life.
- Phenotypically normal males can have two or even three Y-chromosomes.
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Description
Learn about the characteristics and causes of Trisomy 21, also known as Down Syndrome, a common chromosomal disorder influenced by maternal age.
