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

Which component is recorded when determining the modal number of chromosomes?

1. Only the largest chromosomes

2. All chromosomes present

3. A marker chromosome that has a centromere

4. Only pairs of homologous chromosomes

The correct answer is: A marker chromosome that has a centromere

When determining the modal number of chromosomes, the focus is on identifying the most common chromosome count present in a given cell population. The modal number is established by analyzing all the chromosomes to accurately reflect the typical number for that organism or species. A marker chromosome is relevant here because it indicates the identification of particular genetic features that may distinguish it from others, often utilized in cases where there is a deviation from the normal karyotype. This can be particularly important in cytogenetic evaluations where abnormalities or specific mutations in chromosome structure are being investigated. A marker chromosome, which has a discernible centromere, may play a role in understanding these variations. In assessing the modal number, it's crucial to record the count of all chromosomes present; however, the intent of the question and the focus on the specific role of a marker chromosome makes it significant for recognizing abnormalities that may affect the overall count or structure of the chromosomes in the karyotype. Other choices do not adequately represent the entire chromosome population necessary to determine the modal number. By excluding certain chromosomes, such as only focusing on the largest or only homologous pairs, the analysis would fail to capture the complete genetic makeup required for an accurate assessment.