Which translocation is most commonly associated with Down Syndrome?

Down syndrome is primarily caused by the presence of an extra copy of chromosome 21, a condition known as trisomy 21. This can occur through several mechanisms, one of which is a Robertsonian translocation. The most common translocation associated with Down syndrome involves chromosome 21, specifically a translocation between chromosome 21 and another acrocentric chromosome, usually chromosome 14. This results in a viable gamete with an extra chromosome 21 material.

In the context of the provided choices, the translocation involving 21 and 21 refers to the occurrence of a complete trisomy 21 (or Down syndrome), not a translocation. It’s worth noting that while individuals can have a balanced translocation involving these chromosomes, it does not lead to the most frequent type of Down syndrome.

Translocations involving other chromosome combinations do not directly lead to Down syndrome, which is why the translocation involving 14 and 21 is established as the most common genetic variation associated with Down syndrome. This understanding is critical for genetic counseling and assessing risks for families with a history of Down syndrome.