​The mechanism of primary sex determination is best known in Drosophila and mammals. Given below are statements regarding sex determination in these two model systems:

A. In Drosophila, if Sry gene product is present, it may block β-catenin signaling and, along with SF1, activate the Sox9 gene.

B. In mammals, alternate splicing of Sxl transcript that removes a stop codon and allows the formation of a functional protein is responsible for initiating female sex determination.

C. A trans-splicing event in Tra transcript results in the formation of functional Tra protein in Drosophila.

D. XO individuals in Drosophila are males, while XXY individuals are females.

Which of the above statements are correct?

Correct option is B

​Explanation:

Statement A: Incorrect

• Sry (Sex-determining region Y) is present in mammals, not Drosophila.
• In mammals, Sry activates Sox9, leading to male development, but this pathway does not exist in Drosophila.

Statement B: Incorrect

• In Drosophila, Sex-lethal (Sxl) determines female sex by controlling Transformer (Tra) splicing.
• However, in mammals, sex determination is driven by Sry and Sox9, not Sxl.

Statement C: Incorrect

• In Drosophila, Sex-lethal (Sxl) activates Transformer (Tra) via alternative splicing (not by trans-splicing)
• Functional Tra activates Doublesex (Dsx), leading to female development.

Statement D: Correct

• In Drosophila, sex is determined by the X-to-autosome (X:A) ratio, not by sex chromosomes alone.
• XO individuals (X:A = 0.5) are males because they have a single X chromosome.
• XXY individuals (X:A = 1.0) are females because they have two X chromosomes.

Information Booster:

Sex Determination in Drosophila

• Based on the X-to-autosome (X:A) ratio, not sex chromosomes.
• XX (X:A = 1.0) → Female, XO (X:A = 0.5) → Male.
• Sxl (Sex-lethal) gene controls female development by activating Tra (Transformer) via alternative splicing.

Sex Determination in Mammals

• Controlled by the Sry gene on the Y chromosome, which activates Sox9, leading to testis development.
• XX → Female, XY → Male.
• Tra is activated by alternative splicing, not trans-splicing.

Trans-splicing occurs in organisms like trypanosomes, not in Drosophila.

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