Androgen insensitivity syndrome is a genetic disorder, in which a person with an XY genotype - genetically a male – is “insensitive” or doesn’t respond to androgens, which are male sex hormones.
Androgens are responsible for primary sex characteristics like development of the penis and testes as well as secondary sex characteristics like height and body shape, so in androgen insensitivity syndrome all of these can be affected.
Okay, normally, very early on in fetal life, male and female genital tissues are undifferentiated and look identical.
During the 7th or 8th week, the fetal gonads develop either into testes or ovaries.
In males, a gene on the Y chromosome, called the sex-determining region Y gene, or SRY gene for short, helps the fetal gonads turn into the testes.
By the end of week 8, the testes start producing androgens, the main one being testosterone.
A small fraction of testosterone, gets converted by the enzyme 5α- reductase into its more potent form, called dihydrotestosterone, which is mostly responsible for development of male external genitalia.
Looking closely at these structures, at the top there’s the genital tubercle, which is a small projection.
Just below that, there's the urethral groove, which is the external opening of the urogenital sinus or the future urethra and bladder and is surrounded by the urethral folds and the labioscrotal swellings.
Now, once dihydrotestosterone reaches the undifferentiated external genital structures, it makes the genital tubercle elongate into the phallus that eventually becomes part of the penis.