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
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.
In XY individuals, the presence of the Sry gene on the Y-chromosome is responsible for the development of male gonads. This gene encodes a transcription factor called Sry protein, which activates the Sox9 gene. The Sox9 gene, in turn, triggers the expression of testis-forming genes and blocks the expression of ovary-forming genes, leading to the development of male gonads.
If 15% of XY females have the Sry gene, but still phenotypically develop into females, it suggests that the Sry gene is not expressed in these individuals. The absence or non-expression of the Sry gene prevents the activation of the Sox9 gene and subsequent development of male gonads.
Therefore, the correct answer is option d) The Sry gene is not expressed in XY females.
To determine the correct answer, let's break down the given information and analyze each option:
Option a: Normal Sry protein is produced, which transcriptionally activates the Sox9 gene.
This option suggests that the presence of the Sry gene on the Y-chromosome leads to the production of normal Sry protein, which then activates the Sox9 gene. Sox9, in turn, triggers the expression of testis-forming genes while blocking the expression of ovary-forming genes. If this option is correct, it means that the XY females with the Sry gene should develop male gonads and exhibit male phenotypic characteristics.
Option b: Although normal Sry protein is expressed, the gonads are able to express higher levels of the ovary-forming genes.
This option indicates that even though the Sry gene is present and normal Sry protein is produced, the expression of ovary-forming genes is not sufficiently suppressed. Consequently, the gonads develop as ovaries, leading to a female phenotype. If this option is correct, it means that the presence of the Sry gene doesn't fully inhibit the expression of ovary-forming genes in XY females.
Option c: There are mutations in the Sry gene so that it encodes for a non-functional protein.
This option suggests that the Sry gene in XY females has undergone mutations, resulting in the production of a non-functional Sry protein. If this is the case, the absence of functional Sry protein would prevent the activation of Sox9, leading to the development of ovaries and a female phenotype.
Option d: The Sry gene is not expressed in XY females.
This option states that the Sry gene is not expressed at all in XY females. If this is correct, the absence of Sry protein and subsequent lack of activation of Sox9 would result in the development of ovaries and a female phenotype.
After analyzing these options, let's consider the given information once again. It is stated that only 15% of XY females have the Sry gene. This implies that a significant number of XY females do not possess the Sry gene, meaning options a, b, and c cannot explain why these individuals develop into females. Therefore, the most likely answer is option d: The Sry gene is not expressed in XY females. This suggests that the absence of the Sry protein in these XY females prevents the activation of Sox9 and subsequently allows the expression of ovary-forming genes, leading to the development of ovaries and a female phenotype.