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Effects of 5 Alpha Reductase on internal and external genital development

5 Alpha reductase is needed to convert testosterone into dihydrotestosterone.

Testosterone in the fetus acts to induce differentiation of Wolffian (Mesonephric) ducts into the male internal structures, namely, the Seminal Vesicles, Epididymis, Ejaculatory duct, and Ductus deferens (SEED mnemonic).

While dihydrotestosterone acts on the genital tubercle to induce the development of the glans penis and on the urogenital sinus to induce the development of other external structures the prostate gland.

So testosterone ---> Internal structures EXCEPT prostate
while dihydrotestosterone ---> External structures and Prostate

If you don't have dihydrotestosterone, the female external genitalia will develop as they are the default and they don't need a hormone to differentiate.

Therefore, a person with 5 alpha reductase deficiency will develop internal male structures (testicles will be there in the abdomen) and female external genitalia (therefore these Y chromosome individuals are often raised as females).
 

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Awesome explanation Lee. To add to that, another name for 5 alpha reductase deficiency is "penis at 12 syndrome". Basically, although the child is XY genotype they have female external genitalia and so are raised as girls. Then at around 12 years old when they hit puberty, the excess testosterone overcomes the absence of dihydrotestosterone and they get clitoral enlargement, hence "penis at twelve". Usually the child is then taken to the doctor, where they find and remove the undecended testes.
 

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i like to add 2 points;
1-newborne can have female external genitalia OR ambiques genitalis.so we should put this in DDDX of ambiguous genitalis ( male pseudohermapherodism)
2-one more thing,DHT is also responsible for cowper glands developement.
 
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