MCAT Biology Question 180: Answer and Explanation
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Question: 180
15. A child is born with a number of unusual phenotypic features and genetic testing is performed. The child is determined to have partial trisomy 21, with three copies of some segments of DNA from chromosome 21, and partial monosomy 4, with only one copy of some segments of DNA from chromosome 4. Which of the following mutations could have occurred in one of the parental gametes during development to explain both findings?
- A. Deletion
- B. Insertion
- C. Translocation
- D. Inversion
Correct Answer: C
Explanation:
This scenario—a deletion of some DNA and a duplication of other DNA—would be consistent with a translocation between chromosomes 4 and 21 during development of an egg or sperm. If part of chromosome 21 was swapped with part of chromosome 4, then a gamete resulting from meiosis in this cell would result in a daughter cell with two copies of some of the DNA from 21 and no copies of some of the DNA from 4. Therefore, after fertilization, there would be partial trisomy 21 and partial monosomy 4. While a deletion or insertion could explain one of the findings, it cannot explain both, eliminating choices (A) and (B). An inversion should not lead to partial trisomy or partial monosomy because the DNA is simply reversed, eliminating choice (D).