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

What happens to the modal number in a patient with Turner syndrome when an isochromosome is present?

1. It decreases by one

2. It remains the same

3. It increases by two

4. It changes to a completely different number

The correct answer is: It remains the same

In Turner syndrome, individuals typically have a missing or partially missing X chromosome, leading to a common karyotype of 45,X. An isochromosome occurs when one chromosome has an identical arm duplicated, often resulting in an additional copy of one chromosome's genetic material. In the case of Turner syndrome, if a patient has an isochromosome of the X chromosome (i.e., 46,isoXq), the total chromosomal count will still include the original number of chromosomes, maintaining the modal number of 45. The presence of an isochromosome does not change the total number of unique chromosome types in the karyotype, as it essentially replaces one of the normal X chromosomes with a defectively formed one. Therefore, the modal number remains unchanged despite the structural alteration, which is why the answer is that it remains the same. This distinction is important in understanding chromosomal abnormalities and their impact on karyotyping in cytogenetics.