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

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What is the modal number for a diploid karyotype that includes an additional CBG negative marker chromosome?

The correct answer is: 46

In a diploid karyotype, the normal modal number for a human is typically 46, which consists of 23 pairs of chromosomes. When assessing the modal number with the inclusion of an additional chromosome, such as a marker chromosome, it's important to consider the impact of that additional chromosome on the total count. A marker chromosome is an extra chromosomal segment that can vary in size and shape and may not be fully characterized like standard chromosomes. However, it still contributes to the overall chromosome count. In this scenario where there is a diploid karyotype plus an additional CBG negative marker chromosome, you start with 46 and add one more for the marker chromosome. Thus, the total becomes 47. Therefore, the modal number for this situation is 47, indicating that the presence of that additional chromosome has increased the regular chromosome count. This is crucial for assessing genetic conditions, as the presence of extra chromosomes can lead to various phenotypic manifestations.