The SRY gene (Sex-determining Region Y) is a gene located on the Y chromosome. It plays a decisive role in sex development in humans and many other mammals.

• Location: On the short arm of the Y chromosome (Yp11.3).
• Function: The gene encodes a protein (the SRY transcription factor) that acts like a “switch.”
• Effect:
  • Activates other genes that lead to the development of the testes.
  • In the developing testes, testosterone is produced, which in turn controls the formation of male sexual characteristics.
• If the SRY gene is missing (e.g., on an X chromosome or due to a mutation), an organism usually develops a female phenotype despite having XY chromosomes.
• Special cases:
  • Mutations in the SRY gene can cause disorders of sex development (DSD), e.g., Swyer syndrome (XY gonadal dysgenesis).
  • In rare cases, the SRY gene can also be transferred to an X chromosome → resulting in XX males.

The SRY gene is the “start button” for male development in the embryo.

Why a chromosome test is not always sufficient in sports

A simple chromosome test (XX or XY) is not always enough to clearly determine biological sex for sporting purposes. Reasons include:

1. SRY translocation (XX males)
   • Normally: XY → SRY present → male development.
   • But: In rare cases, the SRY gene can be transferred to an X chromosome.
     • Result: XX individual with male development.
     • A chromosome test would only show “XX” → apparently “female,” even though the person has male characteristics.
2. XY without SRY (Swyer syndrome)
   • Some people have XY chromosomes, but due to a mutation lack a functional SRY gene.
     • Result: Female development (no testes, female phenotype).
     • A chromosome test would show “XY = male,” although the person is phenotypically female.
3. Sporting fairness
   • The boxing federation (as well as other sports organizations) wants to ensure that female athletes do not unknowingly compete with genetic or hormonal advantages derived from male development.
   • Therefore, the presence of the SRY gene is more decisive than chromosome count, since it actually gives the starting signal for male development.
4. Additional factors
   • The hormone profile (especially testosterone level) is also often examined, since this ultimately influences physical performance.
   • Testing only chromosomes would therefore be too rough and error-prone.

An SRY test is more precise because it checks whether the “male development switch” was activated – something a chromosome test alone cannot reliably show.

The SRY gene is not always functionally active and not automatically associated with increased androgen production.

1. SRY is only a starting switch

• The SRY gene is activated only briefly in the early embryo (around the 6th–7th week of pregnancy).
• It initiates testis development by activating other genes (e.g., SOX9).
• Later, SRY is not kept permanently active; testis development continues via other regulatory networks.

2. Not always functional

• Mutations in the SRY gene can lead to loss of function, even if the gene is present.
  • Consequence: XY karyotype, but no normal testis development → Swyer syndrome (46,XY gonadal dysgenesis).
• This means: SRY ≠ automatic male development.

3. Androgen production depends on functional testes

• Even if SRY is correctly activated:
  • It must lead to the formation of functional testes.
  • Only the testes produce testosterone (androgens), which then drive male sexual development.
• Disturbances “after” SRY (e.g., defects in testosterone synthesis or in androgen receptors) can still lead to atypical development (e.g., androgen insensitivity syndrome, AIS).

4. Conclusion

• SRY = starter switch, not engine.
• Without functional SRY → no testes → no androgens.
• With SRY, but defective testes or receptors → possibly still low or ineffective androgen levels.
• Only with functional SRY + intact testis development + intact hormone action → typical male androgen production.

Distinction: Presence vs. functionality of SRY

1. Occurrence of SRY mutations

• In the vast majority of cases, the SRY gene is functional.
• Inactive/defective only in very rare cases:
  • Estimates: about 10–15% of cases of 46,XY gonadal dysgenesis (Swyer syndrome) are due to SRY mutations.
  • Swyer syndrome itself is extremely rare: about 1 case per 80,000–100,000 births.
  • This means: only a tiny fraction of all XY individuals have an inactive SRY.

2. Other causes of missing male development despite XY

• Even if SRY is intact, errors further down the pathway can occur, e.g.:
  • Mutations in SOX9, SF1, DAX1
  • Defects in testosterone synthesis
  • Mutations in the androgen receptor (→ AIS)
• These are more common than direct SRY mutations.
• Inactive/mutated SRY gene: extremely rare, <0.01% of all humans.
• Among people with DSD, defective SRY accounts for only a small proportion.
• In the vast majority of cases, SRY is functionally active – problems arise elsewhere in the sex development pathway.

1. Is testosterone the key to athletic performance?

• No, not alone.
  Testosterone plays an important role:
  • Increases muscle mass and strength (anabolic effect).
  • Promotes red blood cell formation (→ more oxygen transport).
  • Influences aggressiveness and risk-taking, which may help in competition.
• But: Athletic performance is multifactorial.
  It depends on:
  • Genetics (muscle type, metabolism, heart-lung function)
  • Training
  • Nutrition
  • Environment (altitude, climate, culture, resources)
  • Psychological factors

2. Could other genetic variations play a role, e.g., in endurance?

• Yes, many genes are associated with endurance performance:
  • ACTN3 gene: influences muscle fiber type (fast vs. slow-twitch).
  • EPO gene & HIF-1α: regulate oxygen transport and adaptation to altitude.
  • ACE gene (angiotensin-converting enzyme): certain variants promote endurance, others power.
• There is no single “super-gene,” but many small contributions (polygenic basis).

• • Why do so many top marathon runners come from East Africa?

• A striking number of world-class runners come from Kenya (Kalenjin people) and Ethiopia.
• Possible reasons (combination of genetics, environment, culture):
  • Genetic factors:
    • Some studies point to favorable combinations in muscle metabolism, VO₂max, and energy efficiency.
    • BUT: No single “marathon gene” has been found.
  • Environment:
    • Living and training at high altitude → natural endurance stimulus (more red blood cells).
  • Sociocultural factors:
    • Running is part of daily life (to school, work, etc.).
    • Strong motivation: running offers economic opportunities.
  • Body build:
    • Lean bodies with long legs and low muscle mass → more efficient energy use over long distances.

4. Conclusion

• Testosterone: important, but not the only factor.
• Athletic performance: result of many genes + environment + training.
• African marathon runners: do not have a “magical genetic advantage,” but benefit from a unique mix of body build, lifestyle, altitude, and training tradition.

• Strength sports: where testosterone is crucial.
• Endurance sports: where many other factors are more decisive.