Molecular Genetics Practice Exam

Which statement best describes dosage compensation mechanisms in mammals and Drosophila?

Mammals upregulate the single X in males via the MSL complex; Drosophila inactivate one X. Mammals inactivate one X via XIST-mediated silencing; Drosophila upregulates the single X in males via the MSL complex. Dosage compensation applies only to autosomes. Dosage compensation always doubles X expression in both sexes.

Correct Answer:

Mammals inactivate one X via XIST-mediated silencing; Drosophila upregulates the single X in males via the MSL complex.

Explanation:

Dosage compensation equalizes X-linked gene expression between males and females. In mammals, females silence one X chromosome through XIST RNA–mediated silencing, forming a Barr body so only one X remains active per cell. In Drosophila, males compensate by upregulating transcription from their single X chromosome via the MSL complex, boosting expression to match females that have two Xs. This combination—XIST-mediated silencing in mammals and MSL-mediated upregulation in male Drosophila—best describes how each sex achieves balance. Dosage compensation targets the X chromosome, not autosomes, and it doesn’t simply double X expression in both sexes.

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