10.1 Repro_Sexual differentiation & reproductive tract

Three Types of Sexual Differentiation

genetic sex (chromosomes),

gonadal sex (type of gonad formed),

phenotypic sex (internal and external genitalia)

Analogy: Blueprints (genes) decide the factory (gonads), which makes products (hormones) that shape the final building (body).

Genetic Sex (XX vs XY)

Complex: Genetic sex is determined at fertilization by sex chromosomes (XX female, XY male), but phenotype depends on downstream gene expression and hormones.
Simple: XX or XY is decided first, but it doesn’t finish the job.
Analogy: Choosing a team doesn’t decide the game outcome.
Sample exam question: Is genetic sex alone enough to determine phenotypic sex? Answer: No.

Gonadal Sex and the SRY/TDF Switch (WEEKS 6–9)

Complex: All embryos start with indifferent gonads and both Wolffian and Mullerian ducts; the SRY gene on the Y chromosome produces testes-determining factor (TDF) at weeks 6–7, causing testes formation, while absence of TDF leads to ovarian development by week 9.
Simple: SRY = testes; no SRY = ovaries.
Analogy: A master switch turns one pathway on and the other off.
Sample exam question: What does the SRY gene do? Answer: Produces TDF to initiate testes development.

Duct Development: Wolffian vs Mullerian

Complex: Testes produce testosterone and anti-Mullerian hormone (AMH); testosterone maintains Wolffian ducts (male internal tract), while AMH causes Mullerian duct regression; without these, Mullerian ducts form female internal organs and Wolffian ducts regress.
Simple: Testosterone saves male ducts; AMH removes female ducts.
Analogy: Keeping one road open while closing the other.
Sample exam question: What happens to Mullerian ducts in a male fetus? Answer: They regress due to AMH.

Phenotypic Sex: Internal vs External Genitalia

Complex: Internal male genitalia (epididymis, vas deferens, seminal vesicles, ejaculatory ducts) require testosterone, while external male genitalia and prostate require dihydrotestosterone (DHT) via 5-α-reductase and androgen receptors; female external genitalia develop by default without estrogen.
Simple: Testosterone builds inside; DHT builds outside (male).
Analogy: Different tools are needed for inside wiring vs outside walls.
Sample exam question: Which hormone is required for male external genitalia development? Answer: DHT.

Default Female Pathway

Complex: In the absence of testicular hormones (testosterone, AMH, DHT), phenotypic sex develops as female regardless of genetic sex, with Mullerian ducts forming fallopian tubes, uterus, cervix, and upper vagina.
Simple: No male hormones = female development.
Analogy: If no one redirects traffic, cars follow the default route.
Sample exam question: What phenotype develops without testosterone and AMH? Answer: Female.

Testes Descent and Timing

Complex: Testosterone induces descent of the testes into the scrotum, usually completed by the 7th month of gestation.
Simple: Testosterone helps testes move down.
Analogy: An elevator lowering to its final floor.
Sample exam question: When is testicular descent usually complete? Answer: By the 7th month of gestation.

Key Abnormalities (HIGH-YIELD)

Complex: Swyer syndrome (XY gonadal dysgenesis) results from SRY mutation causing an XY female phenotype, while Turner syndrome (XO) involves missing X chromosome material leading to gonadal dysgenesis in females.
Simple: Broken SRY = XY female; missing X = Turner syndrome.
Analogy: Missing or faulty instruction manuals change the outcome.
Sample exam question: An XY individual with female phenotype most likely has which condition? Answer: Swyer syndrome.

One-Line Male vs Female Summaries (MEMORIZE)

Male: XY → SRY/TDF → testes → testosterone + AMH → Wolffian ducts + male phenotype; DHT for external genitalia.
Female: XX → no SRY → ovaries → no testosterone/AMH → Mullerian ducts + female phenotype.
Sample exam question: Which single factor initiates male sexual differentiation? Answer: SRY gene producing TDF.