This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Molecular Genetics, Subject name: Biochemistry
[{'content': 'Which one of the following observations would rule out a sex- linked trait in an extended family pedigree? \nSelect and explain. \nA. Males expressing the disease, B. Females expressing the disease, C. Female-to-male transmission, D. Male-to-male transmission', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Sex-linked traits are passed via the X chromosome. Males transmit their Y chromosome to their sons and their X chromosomes to their daughters. Thus, an affected male cannot transmit a mutated X allele to his son, so the presence of male-to-male transmission in a pedigree categorically eliminates X-linked transmission as the genetic pattern of inheritance.', 'role': 'assistant'}]