This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'Chromosomal analysis of all of the following genotype shows Barr bodies, EXCEPT: \nSelect and explain. \nA. XO, B. XXY, C. XX, D. XXX', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Routine microscopy of mammalian cell nuclei reveals a small, dense mass of heterochromatin present in females but not males. This "sex chromatin" (or Barr body or X-chromatin) is one of the two X chromosomes present in females. As a general rule, only 1 X chromosome functions in a cell at a given time. All other X chromosomes present in a cell may be seen as X-chromatin bodies in a resting nucleus. Thus, if one knows the number of X chromosomes, one can anticipate that the number of Barr bodies will be 1 less. If one counts the number of Barr bodies, the number of X chromosomes can be determined by adding 1. Hence Barr bodies are not seen in genotype XO. The male cell has one X chromosome and one Y chromosome, therefore no sex chromatin is visible in males. Ref: Manipalviratn S., Trivax B., Huang A. (2013). Chapter 3. Genetic Disorders & Sex Chromosome Abnormalities. In A.H. DeCherney, L. Nathan, N. Laufer, A.S. Roman (Eds), CURRENT Diagnosis & Treatment: Obstetrics & Gynecology, 11e.', 'role': 'assistant'}]