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MCAT Biology Question – Sex Chromosomes

In the ZW sex-determination system for birds, males (ZZ) are the homogametic sex while females (ZW) are the heterogametic sex. What is the most likely regulatory mechanism for the potential imbalance in gene expression between male and female birds?

A.    Post-transcriptional modification of half the male sex chromosomes gene products

B.    Inactivation of one Z chromosome in males

C.    Inactivation of the W chromosome in females

D.    Destruction of one Z chromosome in males

Click for Explanation

This question asks us to apply knowledge of sex chromosome inactivation in humans to an analogous situation in birds.

Opposite to male birds (ZZ), mammalian males (XY) are heterogametic and carry one X and one Y chromosome. Similarly, female birds (ZW) are heterogametic while mammalian females (XX) are homogametic.

In the mammalian XY sex-determination system, females randomly inactivate one of their X chromosomes to prevent a redundancy in gene products (i.e. mRNA and proteins). These inactivated chromosomes are known as Barr bodies.

In the case of bird species, the homogametic males (ZZ) must inactivate one of their Z chromosomes to achieve the appropriate reduction in gene products. Therefore, the correct answer is B.

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