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

Which characteristic is essential for a chromosome to be included in the modal number?

1. Presence of staining

2. Presence of a centromere

3. Length of the chromosome

4. Region of the genome

The correct answer is: Presence of a centromere

The presence of a centromere is essential for a chromosome to be included in the modal number. The centromere is a key structural feature of chromosomes that plays a crucial role during cell division by ensuring proper segregation of chromosomes into daughter cells. Chromosomes are defined by their centromeres, as they determine the formation of sister chromatids and the overall shape of the chromosome. In cytogenetic studies, the modal number refers to the most frequently observed number of chromosomes in a given set or sample. For a chromosome to be counted toward this number, it must be recognizable as a distinct chromosome, which is facilitated by the presence of a centromere. Without a centromere, a structure may not be adequately identified or classified as a chromosome within the modal analysis. While staining, chromosome length, and regions of the genome provide valuable information regarding the characterization and identification of chromosomes, they are not definitive criteria for establishing whether a structure qualifies as a chromosome in the context of determining the modal number. Thus, the presence of a centromere is the critical characteristic that distinguishes chromosomes and enables them to be included in the modal assessment.