of the Reproductive System
So far, we have described the reproductive organs as they exist
and operate in adults. Now we are ready to look at events that
cause us to become reproductive individuals. Tese events begin
long before birth and end, at least in women, in late middle age.
Embryological and Fetal Events
Discuss the determination of genetic sex and prenatal
development of male and female structures.
Determination of Genetic Sex
Aristotle believed that the “heat” of lovemaking determined
maleness. Not so! Genetic sex is determined at the instant the
genes of a sperm combine with those of an ovum, and the de-
termining factor is the
each gamete contains.
Of the 46 chromosomes in the fertilized egg, two (one pair) are
sex chromosomes. Te other 44 are called
±wo types of sex chromosomes, quite diﬀerent in size, ex-
ist in humans: the large
and the much smaller
. Te body cells of females have two X chro-
mosomes and are designated XX, and the ovum resulting from
normal meiosis in a female always contains an X chromosome.
Males have one X chromosome and one Y in each body cell
(XY). Approximately half of the sperm produced by normal
meiosis in males contain an X and the other half a Y.
If the fertilizing sperm delivers an X chromosome, the fer-
tilized egg and its daughter cells will contain the female (XX)
composition, and the embryo will develop ovaries. If the sperm
bears a Y chromosome, the oﬀspring will be male (XY) and will
develop testes. A single gene on the Y chromosome—the
egion of the
the master switch that initiates testes development and hence
maleness. Tus, the father’s gamete determines the genetic sex
of the oﬀspring. All subsequent events of sexual diﬀerentiation
depend on which gonads form during embryonic life.
However, some of the genes present in autosomes are neces-
sary for a male to become a functional male. Mutation of the
gene can result in XY individuals with the male chromo-
some but female sex characteristics.
When meiosis distributes the sex chromosomes to the gametes
improperly, an event called
, abnormal combi-
nations of sex chromosomes occur in the zygote (fertilized egg)
and cause striking abnormalities in sexual and reproductive sys-
For example, females with a single X chromosome (XO), a
, never develop ovaries. As a
rule, females carrying four or more X chromosomes are mentally
retarded and have underdeveloped ovaries and limited fertility.
, which aﬀects one out of 500 live male
births, is the most common sex chromosome abnormality.
nonindustrialized countries) and respiratory tract inﬂamma-
tions including pneumonia.
Chlamydia can be diagnosed by cell culture techniques and
is easily treated with tetracycline. In 2010, reported cases of
chlamydia in the United States reached a record high of 1.2 mil-
is the most common
S±I in sexually
active young women in the United States. Accounting for about
7.4 million new cases of S±I per year, this parasitic infection is
easily and inexpensively treated. ±richomoniasis is indicated by
a yellow-green vaginal discharge with a strong odor. However,
many of its victims exhibit no symptoms.
due to the
a group of about 60 viruses—is the second most common S±I in
the United States. About 6.2 million new cases of genital warts
develop in Americans each year, and it appears that HPV infection
increases the risk for cancers in infected body regions. Indeed, the
virus is linked to 80% of all cases of invasive cervical cancer. Im-
portantly, most of the strains that cause genital warts do not cause
±reatment is diﬃcult and controversial, and the warts tend to
reappear. Some clinicians prefer to leave the warts untreated un-
less they become widespread, whereas others recommend their
removal by cryosurgery or laser therapy, and/or treatment with
Te cause of
herpes simplex virus 2
these viruses are among the most diﬃcult human pathogens
to control. Tey remain silent for weeks or years and then sud-
denly ﬂare up, causing a burst of blisterlike lesions. Te virus is
transmitted via infectious secretions or direct skin-to-skin con-
tact when the virus is shedding. Te painful lesions that appear
on the reproductive organs of infected adults are usually more
of a nuisance than a threat. However, congenital herpes infec-
tions can cause severe malformations of a fetus.
Most people who have genital herpes do not know it, and
it has been estimated that one-quarter to one-half of all adult
Americans harbor the herpes simplex virus 2. Only about 15%
of that population displays signs of infection.
Te antiviral drug
, which helps the lesions heal
faster and reduces the frequency of ﬂare-ups, is the treatment of
choice. However, once contracted, genital herpes never leaves. It
just goes into periodic remissions.
Check Your Understanding
Which pathogen is most associated with cervical cancer?
What is the most common bacterial STI in the United States?
For answers, see Appendix H.