Mastering Concepts
34.1
1. What is the difference between asexual and sexual reproduction?
Offspring produced by asexual reproduction are essentially identical to their parents,
whereas offspring produced by sexual reproduction are different from each other and
from their parents.
2. How is internal fertilization different from external fertilization?
In internal fertilization, a male copulatory organ delivers sperm into the female’s
reproductive tract. In external fertilization, sperm and eggs are released into the
environment.
3. How do genes participate in differentiation and pattern formation?
Differentiation is the development of cells with specialized functions; pattern formation
is the development of the overall shape and structure of the animal’s body. In both
processes, hormones and other chemical signals activate unique combinations of genes.
These genes encode proteins that produce the unique shapes and functions of specialized
tissues and organs.
4. Differentiate between indirect and direct development.
An animal that undergoes indirect development has a larval stage that looks different
from the future adult. In direct development, there is no larval stage; the young resemble
miniature adults.
34.2
1. What is the relationship among gonads, germ cells, gametes, and the zygote?
Gonads are the reproductive organs that produce the gametes (sperm and egg cells).
Inside the gonads, diploid germ cells undergo meiosis and give rise to the gametes. The
zygote is the fertilized egg.
2. Describe the role of each part of the male reproductive system.
Testes: produce sperm and hormones; scrotum: holds testes; epididymis: stores sperm as
they complete maturation; vas deferens: carries sperm to the urethra; seminal vesicle:
secretes a sugary fluid that is the main component of semen; prostate gland: secretes fluid
that helps to activate sperm; bulbourethral gland: secretes fluid that neutralizes acid left
over from urine; penis: male copulatory organ; urethra: carries semen and urine out of the
body
3. Where in the testes do sperm develop?
Within the testes, sperm develop in seminiferous tubules.
4. What are the stages of spermatogenesis?
A diploid spermatagonium undergoes mitosis to form a diploid primary spermatocyte,
which undergoes meiosis. The product of meiosis I is a pair of haploid secondary
spermatocytes, and the product of meiosis II is four haploid spermatids. The spermatids
mature into sperm cells.
5. What are the parts of a mature sperm cell?
The parts of a mature sperm cell are the head (including the acrosome and nucleus),
midpiece (containing the mitochondria), and tail.
6. How do hormones regulate sperm production?
Gonadotropin releasing hormone (GnRH) from the hypothalamus stimulates the anterior
pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
LH stimulates interstitial cells of testes to release testosterone and other male sex
hormones. Testosterone stimulates sperm production and production of fluid by the
prostate. If sperm production is too high, inhibin temporarily blocks the release of FSH.
34.3
1. What is the role of each part of the female reproductive system?
Ovaries: produce oocytes and hormones; uterine tubes: convey oocyte to uterus; uterus:
nurtures the developing baby; cervix: the opening leading from the uterus to the vagina;
vagina: location where sperm are deposited (the vagina also forms the birth canal)
2. Where do egg cells develop?
Egg cells develop in the ovaries.
3. What are the stages of oogenesis?
A diploid oogonium undergoes mitosis to form a diploid primary oocyte, which
undergoes meiosis. The product of meiosis I is a haploid secondary oocyte (the egg cell)
and a polar body. If the secondary oocyte is fertilized, it completes meiosis II.
4. What is the role of polar bodies in oogenesis?
The role of polar bodies in the maturation of an egg cell is to allow for a single egg cell to
accumulate large amounts of cytoplasm.
5. How do hormones regulate the ovarian and menstrual cycles?
Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the anterior
pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
FSH acts on the ovaries to stimulate the maturation of a follicle. LH and estrogen act on
the ovaries to stimulate ovulation; the follicle then changes into a corpus luteum, which
secretes progesterone that maintains the uterine lining. If fertilization does not occur, the
corpus luteum stops producing estrogen and progesterone, and the uterine lining is shed
in menstruation.
6. List examples of hormone-induced female problems.
Premenstrual syndrome (PMS) accompanies hormonal changes near menstruation;
menstrual cramps result from muscle contractions in the uterus; endometriosis occurs
when displaced uterine cells migrate to the pelvic cavity but continue responding to
hormonal signals; the estrogen withdrawal that accompanies menopause is associated
with hot flashes.
7. Describe some of the most common contraceptives.
Contraceptives include: barriers such as a condom (for the penis or vagina) or a
diaphragm (covering the cervix); spermicidal foam that kills sperm; hormones in birth
control pills, which block ovulation; behaviors like abstinence or the rhythm method (no
intercourse during fertile times); or surgeries like tubal ligations and vasectomies.
34.4
1. List and describe three common sexually transmitted diseases.
One common STD is HPV (human papillomavirus). This virus can cause bumps on the
external genitalia; some strains cause cervical cancer. Another common STD is
trichomoniasis, which is caused by a protozoan. Males are usually symptomless, while
females often produce a vaginal discharge and experience pain and itching. A third STD
is the bacterial disease chlamydia, which often causes pain and burning upon urination.
2. Describe two reasons that a symptomless infection with an STD can be harmful.
A person may not seek medical attention for a symptomless infection, which may
eventually lead to infertility or cancer. Also, a symptomless individual will not know to
abstain from sex and may infect others unknowingly. STD infections also raise the risk of
contracting HIV.
34.5
1. What is the role of the acrosome in fertilization?
During fertilization, the acrosome bursts and releases enzymes that digest the egg’s outer
layers and allow the sperm’s nucleus to enter the oocyte.
2. What is the relationship among the zygote, morula, blastocyst, inner cell mass, and
gastrula?
The zygote is the fertilized egg cell. The morula is a solid ball of cells resulting from the
initial cleavage divisions in the zygote. The morula develops into the hollow blastocyst,
whose inner cells are called the inner cell mass. The inner cell mass develops into the
three-layered gastrula.
3. What is implantation, and when does it occur?
Implantation is the process by which the blastocyst becomes embedded in the
endometrium. It occurs about a week after fertilization.
4. Which supportive structures develop during the embryonic period? What are their
functions?
The yolk sac makes blood cells until the liver takes over this job at the 6th week; the
allantois makes blood cells and produces blood vessels in the umbilical cord; the amnion
cushions and protects the embryo from changes in pressure and temperature; the chorion
helps establish the placenta.
5. When do sex differences appear, and what triggers them?
Sex differences begin to appear after the 7th week of development, when the SRY gene on
the Y chromosome stimulates production of masculinizing hormones in male embryos.
6. What are the events of the second and third trimesters?
In the second trimester, the fetus begins to take on the body proportions of a newborn;
eyebrows, lashes, nipples, and nails develop; bones begin to develop; and the fetus begins
to move around within the uterus. In the third trimester, the brain develops; other organs
differentiate and grow; fat is deposited beneath the skin; and the fetus moves more
vigorously.
7. Which events make up the three stages of labor?
The three stages of labor are dilation of the cervix to 10 cm; delivery of the baby through
the cervix and vagina; and expulsion of the placenta.
34.6
1. What is a birth defect?
Birth defects are those abnormalities that cause death or disability in a child.
2. What are some examples of genetic conditions and teratogens that cause birth defects?
Some examples of genetic conditions that cause birth defects are abnormalities of
chromosome number, such as trisomy 21 (Down syndrome). Faulty genes can also cause
illnesses such as Tay-Sachs disease and phenylketonuria. Teratogens include the
chemicals in alcoholic beverages, tobacco, and drugs.
3. What is the critical period in the development of a structure?
The critical period in the development of a structure is the time when it can be most
easily damaged by a faulty gene, a toxin, or a virus.
34.7
1. List two questions that Hanlon and Naud investigated.
One question was whether males who guard females for longer periods after mating
fertilize more eggs. A second question is whether males that disguise themselves as
females ever fertilize eggs.
2. How would you test the hypothesis that the offspring of a mate-guarding male is more
likely to guard his mate than the offspring of a “sneaker” male? What does this
hypothesis assume about the inheritance of mate guarding?
One experimental design could be to tag male cuttlefish when born as either offspring of
mate-guarding males or “sneaker” males. These offspring could then be observed for
guarding vs. sneaking behavior after they are sexually mature. This hypothesis assumes
that there is a genetic basis to the mate guarding behavior.
Write It Out
1. What are the advantages and disadvantages of asexual and sexual reproduction? Of
internal and external fertilization?
Asexual reproduction produces identical clones of one individual and is advantageous in
an environment that does not change much over time. Sexual reproduction requires two
parents, each contributing to the DNA of the offspring. The genetic diversity in a
population of sexually reproducing organisms increases reproductive success in a
changing environment. In internal fertilization, a male uses a copulatory organ such as a
penis to deposit sperm inside a female’s body. The animals must spend energy to attract
mates, but few gametes are wasted. In external fertilization, males and females release
gametes into the same environment, and fertilization occurs outside the body. The
animals save the energy of finding a specific mate, but many gametes are wasted.
2. How do protein gradients cause cells to differentiate?
Protein gradients cause different parts of an organism to be exposed to different chemical
stimuli. For example, specific proteins concentrating at one end of an organism can
signal the development of the head region, while concentrating another protein at the
other end leads to the development of the hind end. A high concentration of each protein
triggers the expression of a different set of genes, causing cells to become specialized.
Developmental abnormalities can result if proteins are concentrated in the wrong
location.
3. How are the human male and female reproductive tracts similar, and how are they
different? How are the structures of the testis and ovary similar and different?
Each system includes gonads (ovaries in females and testes in males) that contain the
germ cells that give rise to gametes. Each system also includes other sex organs, such as
the network of tubes that transport the gametes; the exact organs differ between the sexes.
Testes and ovaries are similar in that hormones control reproduction; in addition, the
embryological origins of the structures are similar, and the gametes are produced by
meiosis. But the timing and the details of the stages of spermatogenesis are different
from those of oogenesis. Also, sperm develop in tubules, whereas egg cells emerge from
the ovary surface.
4. Make a chart that lists the major hormones involved in human reproduction. Complete
the chart by listing the organ that releases each hormone; the location of target cells; the
function of each; and whether each is most important in males, females, or both.
Hormone
Releasing Organ
Target Cells
Function
GnRH
Hypothalamus
FSH
Anterior
pituitary
Anterior
pituitary
Sustentacular
cells in males;
ovaries in
females
LH
Anterior
pituitary
Testosterone
Testes
Stimulate release of
LH and FSH
Initiates
spermatogenesis in
males; stimulates
follicle maturation in
females
Stimulates testosterone
production in males;
triggers ovulation in
females
Required for sperm
production; stimulates
development of
Interstitial
cells in testes;
ovaries in
females
External
genitalia, hair
follicles,
Male/
Female/
Both
Both
Both
Both
Male
Estrogen
Ovaries
Progesterone Ovaries
larynx,
skeletal
muscle
Ovaries,
uterus, breasts
Uterus
secondary sex
characteristics
Regulates menstrual
cycle; stimulates
development of
secondary sex
characteristics
Prepares endometrium
for pregnancy
Female
Female
5. How are the timetables different for oogenesis and spermatogenesis in humans?
Oogenesis begins before a female is born, halts until puberty, and is not complete until
fertilization. In addition, oogenesis stops entirely at menopause. Spermatogenesis begins
when a male reaches puberty and lasts throughout his life.
6. Point mutations usually occur during interphase of mitosis, but most chromosomal
abnormalities arise during meiosis. Given the differences between gamete production in
males and females, why is it reasonable to predict that more point mutations occur during
sperm production and more chromosomal abnormalities appear in egg cells?
Every spermatogonium undergoes mitosis prior to undergoing meiosis; this process
maintains the number of spermatogonia in the testes. This mitotic division continues
from puberty through life, providing many opportunities for point mutations. In females,
mitosis occurs once in the oogonia prior to birth. From there every primary oocyte in a
follicle is in the process of meiosis, and mitosis will no longer occur. The delay between
the start and end of meiosis in females provides opportunities for chromosomal problems
such as nondisjunction to occur in oogenesis.
7. Is each of the following cell types haploid or diploid? How does each cell type relate to
the others?
a. An oogonium
b. A primary spermatocyte
c. A spermatid
d. A secondary oocyte
e. A polar body derived from a primary oocyte
(a) Oogonium: diploid; (b) primary spermatocyte: diploid; (c) spermatid: haploid; (d)
secondary oocyte: haploid; (e) polar body: haploid. An oogonium is a cell that undergoes
mitosis, giving rise to two primary oocytes; each primary oocyte, in turn, undergoes
meiosis to produce a secondary oocyte and a polar body. A primary spermatocyte
undergoes meiosis I to give rise to two secondary oocytes, which complete meiosis II to
produce four spermatids.
8. How do the structures of the male and female human gametes aid them in performing
their functions?
A sperm cell is small and light, and its flagellum allows it to swim toward the egg cell. Its
head contains enzymes that allow it to deposit its nucleus inside the egg cell. The egg cell
is large and contains the chemicals and organelles needed to start development of the
offspring. Inside the cell membrane are numerous vesicles containing enzymes that
prevent multiple sperm cells from entering the egg.
9. A vasectomy is a surgical procedure in which a physician cuts the vas deferens. Does a
vasectomy stop sperm production entirely, block testosterone production, block sperm
delivery to the ejaculate, or prevent ejaculation? Explain your answer.
Because the testes themselves are not damaged, a vasectomy does not stop sperm
production, but it does block sperm delivery to the ejaculate.
10. Use the Internet to learn more about sexually transmitted diseases. Choose one to
study in detail. What type of infectious agent causes the disease? What are the symptoms
and long-term consequences of infection? Is a treatment available? Who is most affected?
[Answers will vary]
11. What would happen if two sperm fertilized the same egg cell? If two sperm fertilized
two egg cells?
If two sperm fertilized the same egg then the zygote would be triploid (three sets of
chromosomes) and would not be viable. If two sperm fertilized two eggs then both
would implant, and fraternal twins would be produced.
12. How does menstruation stop when a woman becomes pregnant?
During early pregnancy, progesterone prevents menstruation. Specifically, the
preembryo secretes a hormone called hCG, which stimulates cells of the corpus luteum to
continue producing progesterone. High progesterone maintains the uterine lining and
prevents menstruation.
13. This chapter used the term villus in describing part of the chorion; the same term
appeared in chapter 31’s description of the small intestine. How are the chorionic and
intestinal villi similar and different in structure and function?
Intestinal villi increase the surface area of the small intestine; chorionic villi increase the
surface area of exchange between embryo and mother. The functions are different, in
that intestinal villi transfer nutrients from the small intestine to the circulatory system,
whereas the chorionic villi exchange nutrients and wastes between the circulatory system
of the mother and the embryo.
14. Arrange these structures in order from youngest to oldest: morula, gastrula, zygote,
fetus, blastocyst.
From youngest to oldest, the structures are zygote, morula, blastocyst, gastrula, and fetus.
15. Fetal red blood cells contain a version of hemoglobin (an oxygen-binding protein)
that is slightly different from that in adult red blood cells. Given the relationship between
fetus and mother, which hemoglobin do you predict should have a higher affinity for
oxygen? Explain your answer.
Fetal hemoglobin will have a higher affinity for oxygen so that oxygen that was picked
up in the mother’s lungs will cross into the fetal circulatory system at the placenta.
16. Consult a website that describes and illustrates fetal development. What technology
do you think would be necessary to enable a fetus born in the fourth month to survive in a
laboratory setting?
During the fourth month, fetal organ systems have barely begun to develop. The baby
would therefore need an incubator to maintain its temperature. It would also need a way
to deliver oxygen and nutrients to its blood (since the respiratory and digestive systems
are not developed) and to remove wastes. It would also need to be protected from
disease-causing organisms.
17. What are the events of childbirth?
Childbirth begins with labor. The mucus plug is released from the amniotic sac, causing
the amniotic fluid to leak out. Uterine contractions, induced by the presence of hormones,
gradually become more frequent as labor progresses. The cervix will dilate to about 10
cm, and the baby descends the birth canal and is born.
18. List some of the causes of birth defects. What is the significance of the critical period
in determining the type and severity of a birth defect?
Some causes of birth defects include chromosomal abnormalities or faulty genes, alcohol,
cigarettes, drugs, incorrect vitamin levels, starvation, or viral infection. Different organs
undergo their major/important development at different times. This period is termed the
critical period for each organ, and this is when a teratogen will cause the most significant
damage.
19. What kinds of studies and information would be necessary to determine whether
exposure to a potential teratogen during a war can cause birth defects a year later? How
would such an analysis differ if it were a man or a woman who was exposed?
One strategy would be to conduct a statistical analysis of birth defects among children
born to men and women exposed to the suspected teratogen; a control group would
consist of men and women who were not exposed. If birth defects were correlated with
exposure to the potential teratogens in either sex, further study would be warranted.
Experiments on laboratory animals could help confirm that the substance is a teratogen
and determine whether it affects males, females, or both. The substance may affect men
and women differently, because males produce sperm constantly, whereas women are
born with all the immature egg cells they will ever have.
20. Chapter 27’s opening essay describes examples of endocrine disruptors. What are
endocrine disruptors? Would these chemicals be considered teratogens? Why or why not?
An endocrine disruptor alters hormonal signaling by mimicking or blocking a natural
hormone. There is evidence that these disruptors can alter development. If this alteration
results in a birth defect, then they would be teratogens.
21. One risk factor for developing breast cancer or ovarian cancer is a family history of
either disease. Researchers have identified some alleles associated with inherited breast
and ovarian cancers, making genetic tests possible. If a female close to you had a family
history of either cancer, what considerations would help her decide whether to have her
DNA tested for these alleles?
Important questions would include: How many female relatives had the cancer, and what
is their genetic relationship to the woman? Does the cancer occur in both sides of the
family or only one? How strong is the correlation between the alleles and the cancer?
Does the cancer occur in people with one or two copies of the cancer-causing alleles?
The woman’s age, overall health, and personality also would be important factors.
22. Use this textbook or the Internet to learn more about human cloning and stem cell
therapies. How does each topic relate to human reproduction and development? Why are
these techniques controversial?
Human cloning would transfer the diploid nucleus of a donor cell into a de-nucleated egg
cell. The cell would then be stimulated to divide and begin development before being
implanted into a woman’s uterus. Similarly, stem cells are chemically stimulated to
divide and develop into different tissues or organs. Many people feel it is unethical to
clone a human, perhaps because the process is “unnatural” or because the resulting child
may have unforeseen problems. In the same vein, many people feel that destroying a
human embryo to harvest its stem cells is unethical.
Pulling it Together
1. How are spermatogenesis and oogenesis similar, and how are they different?
Spermatogenesis is the development of sperm. Sperm production begins with the onset
of puberty and continues for the rest of a man’s life. It is a constant process, with mature
sperm being continually replenished. Oogenesis is the development of egg cells. Unlike
spermatogenesis, this process begins before birth, halts until puberty, and then resumes
for the duration of the woman’s reproductive life (until menopause).
2. Which hormones influence reproductive function in males and females?
GnRH, LH, and FSH influence reproductive function in both sexes. In men, testosterone
plays an important role in sperm production; in women, estrogen and progesterone help
regulate the ovarian and menstrual cycles.
3. What are the events of fertilization?
Fertilization occurs when a sperm and egg cell fuse, forming a zygote. When a sperm
contacts the outer layer of follicle cells around the egg, the acrosome bursts open to
release digestive enzymes that allow the sperm to penetrate the follicle cells. Then, the
first sperm to penetrate to the egg’s plasma membrane quickly causes the egg to change
its membrane chemistry so no other sperm can enter. The penetrating sperm then
contributes its nucleus to the egg cell.
4. What events occur during each of the main developmental stages?
The preembryonic stage is the development of a single cell into a morula, blastocyst, and
gastrula. During the embryonic stage, the head, limbs, organs, placenta, and other
structures develop. The fetal stage is characterized by further growth and the maturation
of the organ systems. During childbirth, muscle contractions in the uterus push the baby
out of the mother’s body.
5. Add the terms placenta, follicle, polar body, gonad, gamete, meiosis, and mitosis to
this concept map.
The placenta develops during the embryonic stage. Egg cells develop in follicles in the
ovaries. Polar bodies are produced during oogenesis. Testes and ovaries are gonads.
Sperm cells and egg cells are gametes. Both spermatogenesis and oogenesis require
meiosis. The zygote undergoes mitosis as it develops into a baby.