Sex determination in Drosophila is linked to the level of expression of the Sex-lethal gene (Sxl) on the X chromosome. Additional genes that determine sex are transformer (Tra) and doublesex (dsx).
In females with two X chromosomes, Sxl is expressed from an early promoter and act as a splicing factor for Sxl pre-mRNA and tra pre-mRNA. In both cases, exons that contain premature stop codons are spliced out and full length Sxl and Tra proteins are produced. The Sxl mRNA contains exons 1-2-4-5-6-7-8.
Tra protein acts as a splicing factor to make Dsx protein encoded by exons 1-2-3-4 of the dsx gene. This female Dsx protein activates female-specific genes and inhibits male-specific genes for sex determination.
In the male, the Sxl gene is driven by a late promoter, so exon 3 containing a premature stop codon is retained in the mRNA, and truncated, nonfunctional Sxl protein is made. A truncated Tra protein is made since exon 2 with a premature stop codon is not spliced out. A "male” Dsx protein is made from exons 1-2-3-5-6 of the dsx gene and activates male-specific genes for sex determination.
Which of the following statements is correct? Select ALL that apply.
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a
Injection of Sxl protein in an XY male during development would result in the production of female organs
b
Injection of "female" Dsx protein in an XY male during development would result in the production of female organs
c
A Sxl knockout would result in XX flies that have female organs and XY flies that have male organs
d
A Sxl knockout would result in XX flies that have male organs and XY flies that have female organs
e
A Tra knockout would result in XX flies that have male organs and XY flies that have female organs
c
A Sxl knockout would result in XX flies that have female organs and XY flies that have male organs
e
A Tra knockout would result in XX flies that have male organs and XY flies that have female organs
The correct statements are:
b) Injection of "female" Dsx protein in an XY male during development would result in the production of female organs
c) A Sxl knockout would result in XX flies that have female organs and XY flies that have male organs
To answer this question, we need to understand the role of the Sxl, Tra, and Dsx genes in sex determination in Drosophila.
In females with two X chromosomes, the Sxl gene is expressed and acts as a splicing factor for Sxl and Tra pre-mRNAs. It splices out exons containing premature stop codons, resulting in the production of full-length Sxl and Tra proteins. The Sxl mRNA contains exons 1-2-4-5-6-7-8.
The Tra protein, in turn, acts as a splicing factor for the dsx gene, resulting in the production of a female-specific Dsx protein encoded by exons 1-2-3-4. This female Dsx protein activates female-specific genes and inhibits male-specific genes for sex determination.
In males, the Sxl gene is regulated by a different promoter, leading to the retention of exon 3 containing a premature stop codon in the mRNA. This results in the production of a truncated and nonfunctional Sxl protein. The Tra protein is also truncated in males, as exon 2 with a premature stop codon is not spliced out. A "male" Dsx protein is produced from exons 1-2-3-5-6 of the dsx gene, which activates male-specific genes for sex determination.
Based on this information, let's analyze each statement:
a) Injection of Sxl protein in an XY male during development would result in the production of female organs.
This statement is true. If an XY male is injected with functional Sxl protein, it can override the male-specific sex determination pathway and lead to the production of female organs.
b) Injection of "female" Dsx protein in an XY male during development would result in the production of female organs.
This statement is false. The "female" Dsx protein activates female-specific genes, but it cannot entirely override the male-specific sex determination pathway. It may influence certain aspects of sex determination, but it would not result in the production of female organs in an XY male.
c) A Sxl knockout would result in XX flies that have female organs and XY flies that have male organs.
This statement is false. A Sxl knockout would prevent the proper splicing of Sxl and Tra pre-mRNAs, leading to the production of truncated and nonfunctional Sxl and Tra proteins. This disruption in the sex determination pathway would result in XX flies that have male organs and XY flies that are intersex.
d) A Sxl knockout would result in XX flies that have male organs and XY flies that have female organs.
This statement is true. As mentioned in the previous explanation, a Sxl knockout would disrupt the sex determination pathway, leading to the production of male organs in XX flies and female organs in XY flies.
e) A Tra knockout would result in XX flies that have male organs and XY flies that have female organs.
This statement is true. A Tra knockout would prevent the splicing of the dsx gene, resulting in the absence of female-specific Dsx protein. Without the female-specific Dsx protein, XX flies would not develop female organs, and XY flies would develop female organs.
Therefore, the correct statements are:
a) Injection of Sxl protein in an XY male during development would result in the production of female organs.
d) A Sxl knockout would result in XX flies that have male organs and XY flies that have female organs.
e) A Tra knockout would result in XX flies that have male organs and XY flies that have female organs.