genitalia and vagina) is female. It is caused by a mutation in the
androgen receptor gene. Under the inﬂ uence of SRY, the fetal
testes differentiate as usual, and they secrete both MIS and tes-
tosterone. MIS causes the Müllerian ducts to regress, but the
inability of the Wolfﬁ an ducts to respond to testosterone also
causes them to regress, and so no duct system develops. The
tissues that develop into external genitalia are also unresponsive
to androgen, so female external genitalia and a vagina develop.
The testes do not descend, and they are usually removed when
the diagnosis is made. The syndrome is usually not detected
until menstrual cycles fail to begin at puberty.
Sexual Differentiation of the Central
Nervous System and Homosexuality
With regard to sexual behavior, differences in the brain may
form during development. For example, genetic female mon-
keys given testosterone during their late fetal life manifest evi-
dence of masculine sex behavior, such as mounting, as adults.
In this regard, a potentially important difference in
human brain anatomy has been reported: The size of a par-
ticular nucleus (neuronal cluster) in the hypothalamus is sig-
cantly larger in men. A subsequent study showed that the
nucleus is also larger in heterosexual men compared to homo-
sexual men, although there is considerable variability and the
ﬁ ndings should not be overinterpreted. A similar sexually
dimorphic area exists in experimental animals and is known
to be involved in male-type sexual behavior and is inﬂ uenced
during development by testosterone.
Another approach to evaluating the genetics and hor-
mone dependency of sexual behavior and gender preference
is the use of twin and family studies. The pooled data from
many studies show that 57 percent of identical twin brothers
of homosexual men were also homosexual, compared to 24
percent of fraternal twins and 13 percent of nontwin brothers.
The numbers for homosexual women are similar. This sug-
gests a genetic component to sexual orientation.
SECTION A SUMMARY
The gonads have a dual function—gametogenesis and the secretion
of sex hormones.
General Principles of Gametogenesis
I. The ﬁ rst stage of gametogenesis is mitosis of primordial germ
II. This is followed by meiosis, which is a sequence of two cell
divisions resulting in each gamete receiving 23 chromosomes.
III. Crossing-over and random distribution of maternal and
paternal chromatids to the daughter cells during meiosis cause
genetic variability in the gametes.
I. Gender is determined by the two sex chromosomes: Males are
XY, and females are XX.
I. A gene on the Y chromosome is responsible for the
development of testes. In the absence of a Y chromosome,
testes do not develop and ovaries do instead.
• Vas deferens
• Seminal vesicles
• Ejaculatory duct
Differentiation into fetal testes
(on Y chromosome)
• Fallopian tubes
• Inner vagina
• Outer vagina
• Female external
Differentiation into fetal ovaries
Absence of MIS
Absence of testosterone
Sex differentiation. (a)Male. (b) Female. The
gene codes for
the SRY protein. Conversion of testosterone to dihydrotestosterone
occurs primarily in target tissue.
5-alpha-reductase inhibitors, which block the conversion of
testosterone to dihydrotestosterone (DHT) in target tissue, are
used to treat some men with benign swelling of their prostate
glands. (The prostate gland cells contain 5-alpha reductase and
are target tissues of locally-produced DHT.) Examples of these
drugs are ﬁ nasteride and dutasteride. Why are pregnant women
instructed not to take or even handle these drugs?
Answer can be found at end of chapter.