SRY Gene: From Embryology to the Olympics—Why It Matters More Than Ever

The SRY (Sex-determining Region Y) gene has regained importance in elite sports sex eligibility policies, although it is not the sole determinant of biological sex.

Here’s the current picture:

1. Biological role of SRY

• SRY is located on the Y chromosome.
• It encodes the testis-determining factor (TDF), a transcription factor that initiates differentiation of the bipotential gonad into testes by activating genes such as SOX9.
• In embryology, SRY remains the key “switch” for male gonadal differentiation.

2. Why SRY alone is not sufficient

There are important exceptions:

• 46,XY Swyer syndrome – SRY mutation → phenotypic female despite XY chromosomes.
• 46,XX testicular DSD – SRY translocated onto the X chromosome → phenotypic male with XX karyotype.
• Complete Androgen Insensitivity Syndrome (CAIS) – functional SRY and testes are present, but the body cannot respond to androgens, resulting in a female phenotype.

Thus, modern medicine recognizes that genotype, gonadal sex, hormonal function, and phenotype may not always align.

3. Importance in modern sports (Olympics and World Athletics)

SRY has become relevant again because governing bodies are trying to define eligibility for the women’s category.

• World Athletics introduced one-time SRY gene testing (typically by cheek swab or blood sample) for eligibility in the female category beginning in 2025.
• More recently, the International Olympic Committee (IOC) announced that, for future Olympic competition, one-time SRY screening will be used as part of eligibility assessment for women’s events, while recognizing specific exceptions (such as CAIS) rather than treating a positive SRY result as an automatic disqualification.

4. Why the policy is controversial

Scientists and ethicists point out that:

• SRY is an excellent marker of Y-chromosome-associated male sex development, but it does not capture every disorder of sex development (DSD).
• Rare conditions like CAIS or Swyer syndrome illustrate why SRY testing alone cannot define biological sex in every individual.
• Therefore, many organizations use SRY as an initial screening tool, followed by clinical evaluation when necessary.

Pearl:

• Embryology: SRY remains the master sex-determining gene (TDF) initiating testicular differentiation.
• Clinical genetics: SRY is necessary but not always sufficient for male phenotypic development because downstream genes (SOX9, SF1, etc.) and androgen action are also essential.
• Current affairs: SRY has regained significance because it is being incorporated into elite sports eligibility policies by organizations such as World Athletics and, more recently, the IOC