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Chapter 17
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-
nifi
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
cells.
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.
Sex Determination
I. Gender is determined by the two sex chromosomes: Males are
XY, and females are XX.
Sex Differentiation
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.
Development of:
• Penis
• Scrotum
• Prostate
Wolffian ducts
Transformation to:
• Epididymis
• Vas deferens
• Seminal vesicles
• Ejaculatory duct
Müllerian ducts
Regression
Primordial gonads
Differentiation into fetal testes
XY chromosomes
Presence of
SRY
gene
(on Y chromosome)
(a)
Dihydrotestosterone
Müllerian–inhibiting
substance (MIS)
Sertoli cells
Leydig cells
Testosterone
Transformation to:
• Uterus
• Fallopian tubes
• Inner vagina
Development of:
• Outer vagina
• Female external
genitalia
Wolffian ducts
Regression
Müllerian ducts
XX chromosomes
Primordial gonads
Differentiation into fetal ovaries
No
SRY
gene
(b)
Absence of MIS
Absence of testosterone
Figure 17–3
Sex differentiation. (a)Male. (b) Female. The
SRY
gene codes for
the SRY protein. Conversion of testosterone to dihydrotestosterone
occurs primarily in target tissue.
Figure 17–3a
physiological
inquiry
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.
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