2. Sex Determination: reasoning from SRY to phenotype

First principle: one switch, then two hormones

The bipotential gonad defaults to ovary unless SRY diverts it to testis. A testis then does two independent jobs:

The two testicular outputs are separable, and that separability is the clinical reasoning:

• AMH (Sertoli) removes the female ducts;
• testosterone → DHT (Leydig) builds male internal and external structures.

Discriminators: reason from the mismatch

Because chromosomes, gonad and phenotype are built by different steps, a mismatch localises the defect:

Red flags 🚩

• A 46,XY karyotype with dysgenetic (streak) gonads carries gonadoblastoma/malignancy risk → gonadectomy is advised.

Pitfalls

• Equating "Y chromosome" with "male genitalia." Phenotype depends on the whole chain (AMH + androgen + receptor + 5α‑reductase), any link of which can fail.

Pearls

• 🎯 Sertoli → AMH; Leydig → testosterone → DHT. Uterus present vs absent tells you whether AMH was ever active — the fastest split between Swyer and CAIS.

Next → If the Müllerian ducts persist, what exactly do they build — and what stays independent? (Concept 3).