The Genetics of 46,XX Testicular DSD

Understanding the genetics of 46,XX Testicular DSD requires looking past the basic chromosome count. While a standard karyotype—a picture of all 46 chromosomes—can show that an individual has two X chromosomes, it does not have the “magnification” needed to see the tiny genetic switches that actually determine sex development. ^[1][2]

How Testicles Develop with XX Chromosomes

In typical development, the presence of a Y chromosome triggers the formation of testicles. This happens because of a single gene on the Y chromosome called SRY (Sex-determining Region Y). This gene acts like a master power switch. When it turns on, it activates a series of other genes—like SOX9—that instruct the body to build testicles instead of ovaries. ^[3][4]

In 46,XX Testicular DSD, this master switch is flipped even though there is no Y chromosome. This happens in two distinct ways.

Subtype 1: SRY-Positive (The Translocation)

Approximately 80% of individuals with this condition are SRY-positive. ^[5]

• The Mechanism: During the production of sperm in the father, the X and Y chromosomes occasionally swap small pieces of genetic material. This is called translocation. If the SRY gene accidentally moves from the Y chromosome onto the X chromosome, the resulting child will have an XX karyotype but will carry the “master switch” for male development. ^[3][6]
• The Result: Because the SRY gene is present and active, the body follows the male developmental pathway, forming testicles and a male physical appearance. ^[5]

Subtype 2: SRY-Negative (The Bypass)

The remaining 20% of cases are SRY-negative. In these individuals, the SRY gene is completely absent, yet the body still develops testicles. ^[7]

• The Mechanism: This occurs when the genes that normally wait for SRY’s “signal” get turned on by mistake. It is like a backup generator starting up even though the main power switch was never flipped. ^[4][8]
• Key Genetic Players:
  • SOX9 and SOX3 Duplications: Genes like SOX9 are located on completely different chromosomes called autosomes (not the X or Y chromosomes). Because they are on autosomes, they are present in everyone regardless of their karyotype. Sometimes an individual has extra copies (duplications) of these genes or the “enhancers” (on/off switches) near them. These extra copies can be powerful enough to trigger testicular development on their own without the SRY gene. ^[4][8]
  • NR5A1 and WT1 Mutations: Specific changes in these genes can alter the balance of sex development, favoring the male pathway even without SRY. ^[9][10]
  • RSPO1 and WNT4 Loss: These genes normally protect the “female” pathway. If they are not working correctly (loss-of-function), the “male” pathway may take over by default. ^[11][12]

Why Advanced Testing is Essential

A standard karyotype is like looking at a map of a city from a satellite; you can see the big boundaries (the chromosomes), but you cannot see the individual houses (the genes). ^[2]

High-Resolution Microarray

For SRY-negative cases, doctors recommend a high-resolution microarray (often requiring more than 500,000 “probes” or data points). ^[1]

• Detecting “Micro” Changes: This test can find Copy Number Variations (CNVs)—tiny duplications or deletions that are far too small to be seen under a microscope. ^[1][13]
• Precision: It allows doctors to see if those critical “enhancer” regions near the SOX9 gene are doubled, which explains why the male pathway was activated. ^[4]

Whole-Genome Sequencing (WGS)

In complex cases, Whole-Genome Sequencing may be used. This test “reads” every single letter of the genetic code. It is the most thorough way to identify the rare, single-letter mutations in genes like NR5A1 that cause SRY-negative DSD. ^[14][15] Identifying the exact genetic cause is vital for accurate genetic counseling and long-term health planning. ^[14][16]