Which Down Syndrome subtype results from the attachment of chromosome 21 to another chromosome such as 14, 21, or 22?

• A. Trisomy 21
• B. Translocation
• C. Mosaicism
• D. Deletion of chromosome 21

Answer: (B)

Down syndrome can arise from different chromosomal mechanisms that increase the dosage of chromosome 21 genes. This particular pattern is translocation Down syndrome. It happens when the long arm of chromosome 21 attaches to another chromosome through a Robertsonian translocation, often involving chromosome 14, 21, or 22. In a child, you may see a normal number of chromosomes (for example, 46) but with a chromosome that carries extra material from chromosome 21 due to the translocation, giving the effect of an extra copy of chromosome 21. This form can be inherited from a parent who is a balanced translocation carrier, meaning the parent has no symptoms but can pass on an unbalanced version to the child. By contrast, trisomy 21 results from an extra full copy of chromosome 21 due to nondisjunction, mosaicism involves a mix of normal and trisomy 21 cells, and deletion of chromosome 21 involves loss of material rather than gain.