During primary sex determination in mammals, the presence of Sry gene on the Y-chromosome is the major factor in determining if male or female gonads will develop. Expression of Sry protein activates the Sox9 gene, which triggers the expression of testis-forming genes and blocks the expression of ovary-forming genes. XY females are genotypically male but phenotypically female. If 15% of XY females have the Sry gene, why do they phenotypically develop into females?
a
Normal Sry protein is produced, which transcriptionally activates the Sox9 gene.
b
Although normal Sry protein is expressed, the gonads are able to express higher levels of the ovary-forming genes
c
There are mutations in the Sry gene so that it encodes for a non-functional protein
d
The Sry gene is not expressed in XY females
a. Normal Sry protein is produced, which transcriptionally activates the Sox9 gene.
c
There are mutations in the Sry gene so that it encodes for a non-functional protein
The correct answer is d) The Sry gene is not expressed in XY females.
To determine why XY females phenotypically develop into females, we need to understand the process of primary sex determination. In mammals, primary sex determination occurs during embryonic development and is influenced by the presence or absence of the Sry gene on the Y-chromosome.
The Sry gene encodes for a protein called Sry protein. When the Sry gene is present and expressed, the Sry protein activates the Sox9 gene. Sox9 gene activation triggers the expression of genes involved in testis development and inhibits the expression of genes involved in ovary development.
If 15% of XY females have the Sry gene, one might assume that the Sry protein would be produced, leading to the activation of testis-forming genes and the development of male gonads. However, XY females phenotypically develop into females because the Sry gene is not expressed in these individuals.
The absence of Sry gene expression means that the Sry protein is not produced. Without Sry protein, the activation of the Sox9 gene does not occur, leading to the absence of testis-forming gene expression. As a result, ovary-forming genes are not suppressed, leading to the development of female gonads and the phenotypic expression of females.
Therefore, the correct answer is d) The Sry gene is not expressed in XY females.