30. Dosage compensation occurs in-
(1) Insects and mammals
(2) Reptiles and mammals
(3) Reptiles and Aves
(4) Insects and reptiles
Dosage compensation is a genetic mechanism that equalizes the expression of sex-linked genes, primarily on X chromosomes, between males and females. This process is especially significant in organisms where one sex has two sex chromosomes (XX) and the other has only one (XY or X0). For your question, the correct answer is (1) Insects and mammals.
Understanding Dosage Compensation
Dosage compensation is crucial for organisms with differing sex chromosome compositions between males and females. In such cases, there is a risk of gene imbalance, as males (XY) have only one X chromosome, while females (XX) have two. To maintain gene expression balance, insects and mammals have evolved distinct dosage compensation strategies. For example, Drosophila males double the gene expression from their single X chromosome, while mammalian females inactivate one X chromosome in every cell to match gene dosage with males.
Explanation of All Options
(1) Insects and mammals
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Correct: Both groups have well-characterized dosage compensation systems.
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In Drosophila (insect), males upregulate their single X chromosome to match females.
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In mammals, one X chromosome in females is inactivated to balance X-linked gene expression.
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No similar universal, chromosome-wide compensation system is known in reptiles or aves.
(2) Reptiles and mammals
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Incorrect: While some evidence of X chromosome differentiation and gene regulation exists in certain reptiles (such as Anolis lizards), a consistent, chromosome-wide dosage compensation system like those found in mammals and insects does not occur broadly in reptiles.
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Mammals do show robust dosage compensation.
(3) Reptiles and Aves
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Incorrect: Birds (aves) exhibit an incomplete dosage compensation system. In birds, Z chromosome genes are not compensated to the same extent as in mammals or insects, leading to higher dosage in males (ZZ) compared to females (ZW).
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Most reptiles lack established chromosome-wide dosage compensation.
(4) Insects and reptiles
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Incorrect: While insects possess effective dosage compensation strategies, reptiles generally do not show global chromosome-wide dosage compensation.
Dosage Compensation Mechanisms Table
| Group | Dosage Compensation Present? | Mechanism |
|---|---|---|
| Mammals | Yes | X inactivation in females |
| Insects | Yes | Upregulation of male X chromosome |
| Reptiles | Rare/Partial | Not universal; limited evidence |
| Birds | Incomplete | Partial upregulation of Z in females |
Conclusion
Dosage compensation occurs specifically in insects and mammals, with clear, well-studied mechanisms in both groups to ensure balanced gene expression from sex chromosomes. This is why (1) is the correct answer to the question.
