Genetics of Down syndrome

Genetics of Down Syndrome is a topic that explores the genetic factors contributing to the occurrence of Down Syndrome, a chromosomal condition that affects physical and cognitive development.

Overview

Down Syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is typically associated with physical growth delays, characteristic facial features, and mild to moderate intellectual disability. The average IQ of a young adult with Down Syndrome is 50, equivalent to the mental ability of an 8 or 9-year-old child, but this varies widely.

Genetic Causes

The extra chromosome 21 material that causes Down Syndrome may come from either the mother or the father. There are three types of abnormal cell division involving chromosome 21 that can lead to Down Syndrome:

1. Trisomy 21 (nondisjunction): Down Syndrome is usually caused by an error in cell division called "nondisjunction." Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This type of Down Syndrome, which accounts for 95% of cases, is called trisomy 21.
2. Mosaicism: Mosaicism (or mosaic Down Syndrome) is diagnosed when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra chromosome 21.
3. Translocation: Translocation accounts for 4% of all cases of Down Syndrome. Part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down Syndrome.

Risk Factors

The only known risk factor for Down Syndrome is maternal age. Women aged 35 and older have a significantly higher risk of having a child with Down Syndrome than younger women. This is because older eggs have a greater risk of improper chromosome division. A pre-existing abnormality in the parents' chromosomes could also increase the risk of having a child with Down Syndrome.

See Also

• Genetics
• Chromosomal Abnormalities
• Intellectual Disability

References

Genetics of Down syndrome Resources
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