■ SECTION 6.1 ASSESSMENT, p. 203 Check Your Understanding Answers Checking Concepts
1. The number of chromosomes in a human skin cell is 46 (the diploid number) and in a human egg cell is 23 (the
haploid number).
2. To identify a pair of homologous chromosomes, you would look for two chromosomes that have the same size and
shape.
3. The benefits of genetic diversity are that organisms may have new characteristics that allow them to be better
equipped to cope with changes in the environment. The organism may gain an advantage over another organism.
4. For a zygote to become an embryo, the cell must undergo mitosis and cell division.
5. (a) Meiosis II (b) Meiosis I (c) Meiosis I (d) Meiosis I (e) Meiosis II
6.
7. In meiosis I, the homologous chromosomes are paired at the equator. In mitosis,
the homologous chromosomes are unpaired and line up individually at the equator.
8. There is the diploid number (46 chromosomes in humans) in metaphase of mitosis
and the haploid number (23 chromosomes or 46 chromatids in humans) in metaphase
II of meiosis.
9. In crossing over, parts of non-sister chromatids exchange information that could
create new characteristics.
10. In independent assortment, there are two possibilities of how a chromosome will sort itself into the daughter cells.
This is true for each of the 23 pairs, so this leads to many possible combinations.
11. Examples of chromosome mutations can include the loss of a whole chromosome, or part of a chromosome, the
duplication of a chromosome, or the movement of a piece of the chromosome within itself or to another chromosome.
Students might mention fruit fly mutations here as well. Mitosis Meiosis Where does it take place? Unicellular
organisms and body cells of multicellular organisms Sex cells of multicellular organisms How many cells are produced? 2
4 What happens to the number of chromosomes? Remains the same as parent cell Half the number of chromosomes as
original How do parent and daughter cells differ genetically? Identical Not identical because of crossing over and
independent assortment How do daughter cells compare to each other genetically? Identical Not identical because of
crossing over and independent assortment Chapter 6 • MHR TR 2-39
12. Examples of genetic disorders illustrated in the text include: Down syndrome and Edwards syndrome.
Understanding Key Ideas
13. Whole chromosomes can be lost or added in meiosis I when homologous chromosomes fail to separate or in meiosis
II when sister chromatids fail to separate.
14. Meiosis is important for the survival of the organism because the process introduces variation and variation is
necessary for the survival of the species if conditions change.
15. You can tell whether a sperm cell is in meiosis I or meiosis II because in meiosis I whole chromosomes are moving to
the poles. In meiosis II, chromatids are separating and moving to the poles.
16. The picture should be similar to Figure 6.5 on page 191. The two chromosomes should appear to be the same size
and shape.
17. (a) The dog would have 38 homologous pairs of chromosomes. (b) Each sperm cell would have 38 chromosomes.
18. Chromosomes condense and become visible in mitosis so a karyotype would need to be done on a type of cell that is
preparing to divide, such as a skin cell or cheek cell.
■ SECTION 6.2 ASSESSMENT, p. 223 Check Your Understanding Answers Checking Concepts
1. For successful sexual reproduction to occur, two parents of the same species are required, and two gametes must
come together for fertilization.
2. The three stages of sexual reproduction are mating, where gametes are brought together; fertilization, which is the
union of the egg and sperm; and development, which is a series of mitotic and cell divisions of the zygote that form the
embryo and fetus.
3. Answers may vary. Mammals that mate in water include whales, sea lions, sea otters, dolphins, and seals.
4. Internal fertilization occurs when the sperm cells are deposited inside the female’s body where they meet the egg.
Usually only one sperm can fertilize an egg.
5. External fertilization occurs when a sperm and egg unite outside the bodies of the parents.
6. When an egg and sperm unite, this restores the diploid number of chromosomes. If more than one sperm was able to
fertilize the egg, the fertilized egg would have more than the diploid number of chromosomes. The fertilized egg would
be unable to undergo any more cell divisions as the egg would not pass the checkpoints because of the incorrect
number of chromosomes. Chapter 6 • MHR TR 2-43
7. Water or water-containing fluid is necessary for animals that reproduce sexually to prevent the dehydration of the
egg, sperm, and embryo. This is not a problem in water but does become a problem on land.
8. In plants, the pollen tube delivers the sperm to the egg.
9. In flowering plants, the seed is enclosed in a fruit. In cone-bearing plants, the seed is protected within the female
cone.
10. The stages of human development in order are: (b) zygote, (f) morula, (a) blastula, (d) gastrula, and (c) fetus.
11. Answers should be similar to the sketches on page 216. The morula should be a ball of cells, the blastula should
show a hollow ball of cells, and the gastrula should show three layers of cells with a hollow centre.
12. (a) Ectoderm: (i) skin, (ii) nervous system (b) Endoderm: (v) digestive system, (vi) lungs (c) Mesoderm: (iii) skeletal
system, (iv) kidneys
13. The name of the female plant structure that stores the egg cells is the ovule. Understanding Key Ideas
14. Graphic organizers may vary. Sample organizer:
SECTION 6.3 ASSESSMENT, p. 231 Check Your Understanding Answers Checking Concepts
1. Answers will vary and may include: the man is unable to produce enough sperm or the woman is unable to produce
sufficient hormones. In addition, disease, medical treatments such as chemotherapy, or radiation may have damaged
the eggs or sperm.
2. The function of the uterus is to provide a place for the development of the fertilized eggs.
3. The steps of in vitro fertilization are as follows. First, the woman is given powerful hormone treatments to increase
the number of eggs produced. Then, the eggs are taken from the woman and sperm cells from the man and combined
in a petri dish. Two to four days after fertilization, the fertilized embryos are implanted in the uterus.
4. Some potential side effects of in vitro fertilization for the woman are that the hormonal treatment to increase egg
production can cause dizziness, nausea, and headaches.
5. Only 25 births in 100 result from ICSI procedures. (You may wish to mention the statistics on in vitro fertilization—
only 33% of in vitro procedures are successful.)
6. The first in vitro fertilization occurred at the University of British Columbia.
7. Embryos can be preserved for an extended period of time by freezing them in liquid nitrogen in a process called
cryopreservation.
8. A surrogate mother may carry the fertilized egg for a couple, or the surrogate mother may provide the egg for
fertilization as well as carry the embryo. Understanding Key Ideas
9. Some advantages of reproductive technology are that some infertile couples are able to have a child; couples can
select embryos to exclude embryos with genetic defects; parents trying to help cure a sick child can screen and select
embryos for tissue matches; and extra embryos can be made available for stem cell research.
10. Some disadvantages of reproductive technology are that the success rate is not that high considering that only 33%
of in vitro fertilizations are successful, and hormonal treatments can cause discomfort for the woman. Also, ethical
issues arise regarding the unused embryos: some couples might choose to screen embryos for sex selection or for a
desirable trait; frozen embryos may eventually be discarded; and problems arise with surrogacy as people change their
minds about ownership of the child.
11. In gamete intrafallopian transfer (GIFT), many eggs and sperm are combined and immediately put into the female so
that fertilization occurs inside the woman’s body rather than in the petri dish. The embryo is not produced outside the
woman’s body. In intracytoplasmic sperm injection (ICSI), a single sperm in injected into a single egg and fertilization
occurs in the petri dish. Then the embryo is inserted into the uterus of the woman.
12. Check to make sure the students have designed and labelled their graphs correctly (up to 35, 40 out of 100; up to 39,
20 out of 100; up to 43, 10 out of 100).
Reading Check Answers, p. 194
1. Genetic diversity is the inherited differences in a population. For example, not everyone in the classroom is the same
height.
2. The function of meiosis is to reduce the chromosome number so that when the egg and sperm unite the original
number of chromosomes will not be doubled.
3. (a) The haploid number of chromosomes in humans is 23. (b) The diploid number of chromosomes in humans is 46.
4. Another name for a fertilized egg is a zygote.
5. Homologous chromosomes are a matching pair of chromosomes that are the same size and shape. They have genes
coding for the same trait in the same location. (One member of the pair originally came from the mother’s egg and the
other from the father’s sperm).
Reading Check Answers, p. 215
1. Unique proteins and sugars on the surfaces of eggs and sperm help in species identification.
2. The method of fertilization for land-dwelling animals is usually internal fertilization.
3. The method of fertilization for water-dwelling animals is usually external fertilization.
4. Pollination is the method of internal fertilization for land-dwelling plants, where sperm is transferred to the female
productive part of the plant.
5. Within a seed is a plant embryo and stored food for the embryo.
Reading Check Answers, p. 228
1. Assisted reproductive technology describes methods used to assist fertility for couples who are unable to produce a
child.
2. Artificial insemination involves techniques for collecting sperm from a male and injecting it into a female.
3. In the technique of in vitro fertilization, the egg is fertilized in a petri dish and the fertilized egg is then implanted in
the female’s uterus.
4. As a woman ages, the success rate of in vitro fertilization decreases.
5. The difference between gamete intrafallopian transfer and in vitro fertilization is that in gamete intrafallopian
transfer the fertilization occurs inside the woman’s body, not in a petri dish.
CHAPTER 6 ASSESSMENT, p. 232–233 ■ PREPARE YOUR OWN SUMMARY Student summaries should incorporate the
following main ideas: 1. Meiosis • Meiosis is necessary to produce gametes with the haploid number of chromosomes. •
Meiosis creates genetic diversity, which can give one organism an advantage over another. • There are two cell divisions
in meiosis including meiosis I and meiosis II, but the DNA only replicates once. • Chromosome mutation can occur in the
process of meiosis and can create a syndrome in the offspring. 2. Genetic Variation • Crossing over and independent
assortment are events in meiosis I that create variation. • Genetic variation is also created by the chance fertilization of
a certain egg with a certain sperm. 3. Methods of Fertilization in Sexual Reproduction • Sexual reproduction in plants
and animals must occur either by internal or external fertilization. • In external fertilization, the sperm and egg unite
outside the bodies of the parents. • In internal fertilization, the sperm unites with the egg inside the body of the female.
• In plants, internal fertilization occurs following pollination. • There are advantages and disadvantages to internal and
external fertilization. 4. Early Embryonic and Fetal Development • Once the egg is fertilized, certain conditions must be
met if it is to continue to divide to form an embryo. • After several cell divisions, the fertilized egg forms a morula, then
a blastula, then a gastrula. • The gastrula has three layers of cells, which will each differentiate into different body
organs. 5. Types of Assisted Reproductive Technologies • Reproductive technologies assist infertile couples. • In all
these technologies, the sperm is assisted in uniting with the egg either inside or outside the female’s body. Checking
Concepts
1. Meiosis is necessary for multicellular organisms so that the chromosome number can be retained and so that genetic
variation can occur.
2. Meiosis II is very similar to mitosis since in both processes sister chromatids separate.
3. At the beginning of meiosis, the parent cell is diploid. At the end of meiosis, there are four haploid cells.
4. The two events that produce genetic variation are crossing over, where genetic information is exchanged between
non-sister chromatids, and independent assortment, where there are two possibilities for how a chromosome will sort
itself into daughter cells.
5. Crossing over is like shuffling a deck of cards because there are many, many possibilities of how the genetic material
in a gamete will order itself. Crossing over can occur multiple times, like cutting the cards and shuffling a deck of cards.
6. You are not identical to your parents or siblings because of the reshuffling from crossing over and independent
assortment. In independent assortment, the egg contained chromosomes from your grandparents on your mother’s
side, and the sperm contained genetic information from your grandparents on your father’s side. For each chromosome
in the egg, there was a 50% chance that you inherited your grandmother’s genetic information, 50% that the genetic
information was from your grandfather on your mother’s side. This is true for all 23 chromosomes, so this is 223
possibilities just from the egg, and there would be 223 possibilities for the sperm.
7. A karyotype allows geneticists to determine whether there has been a chromosomal mutation. A whole chromosome
could have been deleted or added, or a piece of a chromosome could have been added, deleted, or moved onto
another chromosome.
8. The syndrome with three chromosomes 21s is Trisomy 21, commonly known as Down syndrome.
9. Answers will vary as there are many ways that students could connect these ideas. Check for correct associations.
10. (a) Internal fertilization (b) External fertilization (c) External fertilization (d) Both, since this is sexual reproduction
from the union of the egg and sperm
11. (a) The period of rapid growth rate is from three to six months. (b) The growth is the slowest between the sixth and
seventh month. TR 2-48 MHR • Reproduction (c) The fastest growth rate shows the steepest slope on the graph. The
slowest growth rate is shown in the least steep slope. Understanding Key Ideas
12. The purpose of fertilization is to join the egg and the sperm, to restore the diploid number for the offspring.
13. External fertilization is suited to aquatic environments because the eggs, sperm, and zygote will not dry out, and the
water helps disperse the eggs and sperm.
14. A single fertilized egg grows into a multicellular embryo by dividing millions of times by mitosis. The cells
differentiate and become specialized to perform a particular function. This means certain genes will now be read and
other genes will no longer be read.
15. Pollen is male sperm and therefore contains DNA. Each species would have unique DNA in the pollen that might help
in a crime investigation.
16. (a) 1. Ectoderm; 2. mesoderm; 3. endoderm (b) Answers may vary but should include any one of each of the
following organs. Ectoderm: skin, nervous system; mesoderm: kidneys, reproductive organs; endoderm: lining of
digestive system, lungs, liver
17. Scientists prefer to use embryonic stem cells because these cells have the potential to become any type of cell.
Teenage or adult stem cells can become only a few different types of cells. For example, an adult blood stem cell could
become only blood tissue, not muscle cells or skin cells.
18. The researcher is looking at a karyotype. By doing this analysis, the researcher can determine if the person has a
chromosome mutation.
19. Reproductive technology has an impact on society because it helps infertile couples to conceive. However, the
technology raises many ethical and other questions, such as should these technologies be free and available to
everyone, should we be selecting for embryos, who are the biological and legal parents of the child, and what do we do
with the unused embryos.