American Society for Clinical Pathology (ASCP) Technologist in Cytogenetics certification (ASCP CG) Practice Test

What is the modal number of chromosomes in a cell for a Down syndrome translocation carrier?

1. 46

2. 47

3. 45

4. 48

The correct answer is: 45

In individuals with Down syndrome resulting from a translocation, the modal number of chromosomes can vary based on the specific translocation involved. Typically, this condition arises from the presence of an extra copy of chromosome 21, which contributes to the characteristic features of Down syndrome. For a person who is a translocation carrier of Down syndrome, they often possess a standard set of chromosomes but with a rearrangement involving chromosome 21. Standard individuals have 46 chromosomes; however, in the case of a complete trisomy 21 (the most common form of Down syndrome), there are 47 chromosomes due to the extra chromosome 21. In contrast, a translocation carrier may still have 46 total chromosomes if the extra chromosome material is attached to another chromosome instead of being a separate chromosome. This condition is responsible for a stable chromosome count despite having Down syndrome due to the translocation because the additional genetic material can displace, rather than add to, the overall chromosome count. Thus, the typical modal number for a carrier of a Down syndrome translocation could indeed be 46 because while they carry material that contributes to Down syndrome, their overall chromosome count remains unchanged. Therefore, answering with 45 or 48 does not effectively reflect the biology of the trans