(SPOILER) AAMC FL 3 Q 11

[premed7195]

Assume that a certain species with sex chromosomes R and S exists such that RR individuals develop as males and RS individuals develop as females. Which of the following mechanisms would most likely compensate for the potential imbalance of sex-chromosome gene products between males and females of this species?

• A.
  Inactivation of one R chromosome in males
• B.
  Doubling transcription from the S chromosome in females
• C.
  Inactivation of the R chromosome in females
• D.
  Doubling transcription from the R chromosomes in males

Could someone please explain why is the answer not B? My logic behind choosing B was that since S is in lesser quantity, doubling its expression would match the R expression. I have read about X inactivation in humans but how does that relate to this question?

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[M&L]

The answer is A - imagine that genders are reversed - this is the process of lyonization in females.

 

[JimKimSlim]

It's simple mathematics. We know males have two R chrosomes, while females have only one R chromosome. Choice A - males will produce the same amout of transcriptomes from R chromosome as females who have only one R chromosome. Choice B - Females will produce more transcriptomes from S chromosome, which males don't even have. Choice C - Females will not produce any transcriptome from R chromosome. Choice D - males will produce four times the amount of transcriptomes females produce from a single R chromosome. I hope you see my point.

Another option I was expecting to see was the inactivation of the S chromosome in females, but that would be too hard to guess without more information.