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Show Me the Genes!
REA DI
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ACTIVIT Y OVERVIEW
SUMMARY
A reading describes the behavior of chromosomes during sexual reproduction and its
consistency with basic patterns of inheritance. In addition, the function of DNA and
the effects of randomly occurring mutations are introduced.
KEY CONCEPTS AND PROCESS SKILLS
1.
Sexual reproduction involves the union of two sex cells and produces unique
individuals that show a combination of traits inherited from both parents.
2.
An allele is one of the two or more forms of a gene present in a population.
3.
Genes are located on chromosomes, which are located in the nucleus of every
cell in humans and other organisms with nucleated cells.
4.
The behavior of genes is related to the behavior of chromosomes.
5.
Each individual obtains half of its chromosomes (and roughly half of its genes)
from each parent.
6.
DNA is the molecule that contains the information carried by the genes.
7.
Mutations result when random errors occur during the copying of DNA before
cell division.
KEY VOCABULARY
allele
fertilization
chromosome
mutation
cell division
nucleus
DNA
sperm cell
egg cell
trait
gene
Teacher’s Guide
D-113
Activity 63 • Show Me the Genes!
TEACHING SUMMARY
Getting Started
1.
Discuss students’ ideas about the location of the genes.
Doing the Activity
2.
Students read about and discuss the role of chromosomes in cell division.
3.
Students read about and discuss the role of chromosomes in inheritance.
4.
Students begin to consider the role of DNA mutations in genetics.
Follow-Up
5.
Reflect upon the correspondence between chromosome behavior and the
transmission of genes.
Extension
Students watch a video excerpt that relates genetic mutations to the development of
cancer.
D-114
Science and Life Issues
Show Me the Genes! • Activity 63
BACKGROUND INFORMATION
Chromosomes, Cell Division, and Reproduction
The chromosomal basis of inheritance was first proposed by a young scientist named
Walter Sutton in the early 1900s. His group’s work not only established the chromosomal location of the genes, but also explained why the independent assortment of
many genes (see Mendel’s work) applies only to genes that are located on different
chromosomes. The emphasis of the student reading is that chromosomes occur in pairs
and that for each gene, the two alleles are found on a single pair of chromosomes.
The reading stresses the observation that the total (diploid) number of chromosomes
stays constant during normal cell division (mitosis), but is halved in the formation of
(haploid) egg and sperm cells (meiosis). The details of the stages of mitosis and meiosis are typically taught in high school. Knowledge of the stages is not necessary for an
understanding of the outcomes. However, the “pairs of socks” analogy used in the
reading may help students realize that if two parents, such as Skye and Poppy, each
contribute a tail-color allele to an offspring, they must each contribute a similar chromosome containing that tail-color allele.
Chromosomes can be visible only in cells arrested before cell division, when the chromosomes are in replicated form or are migrating into the two daughter cells, as shown
in Figure 2 on page D-49 in the Student Book. In preparation for either mitosis or
meiosis, the chromatin (DNA and associated proteins) is in condensed form. Note
that the last frame of Figure 4, on page D-50 in the Student Book, shows a pair of chromosomes as they would look immediately after mitosis or the second division of meiosis, i.e. as single chromosomes ready to decondense again. This frame represents not
a “pair of chromosomes,” but two identical copies of a single chromosome. Each copy
will migrate into a separate daughter cell. (In reality, these copies are not truly identical, due to the effects of recombination, which further shuffles gene combination
during sexual reproduction.)
Teacher’s Guide
D-115
Show Me the Genes! • Activity 63
TEACHING SUGGESTIONS
GETTING STARTED
1.
growth and replenishment of dead cells in multicellular organisms. In the second passage, students
explore the appearance of chromosomes under the
microscope and the need for cells to copy their
Discuss students’ ideas about the location
chromosomes before division, so that the number
of the genes.
of chromosomes within cells stays constant.
Begin by asking students if they have any ideas
n Teacher’s Note: Basic cellular division is referred
about where the genes are located. Listen to their
to as mitosis, whereas meiosis is the division process
answers and use them to address misconceptions as
that produces haploid sex cells. Because these terms
the activity proceeds. Scientists about 100 years ago
are not necessary for students to understand the
realized that the sperm contains very little cyto-
basic concept that division to produce the sex cells
plasm—the only common feature of both egg and
results in half of the normal chromosome number
sperm was the nucleus. Therefore, the nucleus
and genes, they are not included as vocabulary. The
seemed a likely candidate for the location of the
steps of mitosis and meiosis are often included in
hereditary material. You may want to ask students,
the high school curriculum and can be explored in
What scientific advances enabled scientists to
more depth at that time.
draw this conclusion? The improvement of microscopes and the development of the cell theory were
precursors to this understanding of fertilization and
the role of the sex cells.
3.
Students read about and discuss the role of
chromosomes in inheritance.
Part Two of the reading looks at the natural oscillation of chromosome number during sexual repro-
DOING THE ACTIVIT Y
2.
Students read about and discuss the role of
chromosomes in cell division.
duction, from body cell to sex cell to fertilization.
Stopping to Think 3 asks students to consider what
would happen to the chromosome number if each
parent contributed all of its genetic information to
This reading is most effective if done as a class read-
the offspring—the number of chromosomes would
aloud, with different students reading different
double with each generation. Students familiar with
paragraphs. Be sure to take plenty of time to discuss
geometric progressions will realize that this would
each figure and its relationship to the reading. Alter-
eventually lead to a vast number of chromosomes—
natively, you can have students read passages aloud
more than any fertilized egg could hold! This is the
within their groups, pausing to discuss each Stop-
essential point for students to understand, as it will
ping to Think question once each group has had a
allow them to relate the behavior of chromosomes
chance to consider it.
to the basic genetic model they’ve been exploring
The first passage clarifies that cell division is the
for several activities.
method of asexual reproduction in single-celled
The next passage of the reading explores an analogy
organisms, but it is also the process that enables
using 23 pairs of socks to represent the chromosomes.
Teacher’s Guide
D-117
Activity 63 • Show Me the Genes!
In responding to Stopping to Think 4, just as in the
Mendelian model they have been exploring, students
should not expect siblings to have the same set of
chromosomes (and thus the same alleles for each
gene). After all, genes are present on chromosomes,
FOLLOW–UP
5.
Reflect upon the correspondence between
chromosome behavior and the
transmission of genes.
and only one chromosome of each pair is given to
Reinforce the parallel between the behavior of the
each offspring. Students are already familiar with this
chromosomes as observed under a microscope and
fact from observations of differences in traits among
the model for the behavior of the pea plant genes
biological siblings. However, urge them to think at
developed by Mendel and simulated for critter
the level of the genes and chromosomes, which will
genes using coin tosses.
be explored more clearly in Activity 65, “Breeding
Critters—More Traits.” The probability of two siblings receiving the same 23 chromosomes from even
one of the two parents is extremely small.
4.
You may want students to go back to Activity 58,
“Creature Features,” to think about whether they
would have considered multiple hypotheses to be
possible if they had had the multiple sources of evi-
Students begin to consider the role of DNA
dence they have since learned about. Why or why
mutations in genetics.
not? What evidence have they discovered that sup-
Part Three of the reading describes the large number
of genes present in organisms, specifically humans,
and the complexity of inheritance for most traits.
You may wish to use a slinky or a diagram of a coil
as a simple model of the structure of DNA. Explain
that this coil is very tightly packed upon itself to
form a chromosome.
Most importantly, Part Three introduces the idea
that mutations are random changes in the DNA
sequence of genes, which can affect the organism.
In discussing Stopping to Think 5, explain that
mutations can occur at any cell division (actually,
during the copying of DNA that precedes it). (See
Extension below.) However, mutations that change
the entire form of an organism occur only during
the formation of sex cells in the parents of that
organism.
D-118
Science and Life Issues
ports a two-allele hypothesis more than any other?
Show Me the Genes! • Activity 63
SUGGESTED ANSWERS
TO ANALYSIS QUESTIONS
1.
Draw a flow diagram (a series of pictures) such
as the one below that shows the locations and relative sizes of DNA, genes, chromosomes, and cells in
a human body. Write a paragraph to explain your
diagram.
■ Teacher’s Note: Suggest to students that they
start with the person, rather than with the
DNA, before they begin the assessment. Students’ diagrams should show each of the
body
requested components, illustrating the cells as
organ system
organ
structures within the person and chromosomes
as structures within the nucleus of each cell.
Each gene is on a chromosome, which is made
of a tightly coiled double-strand of DNA. A sam46 (23 pairs)
chromosomes
in each
nucleus
skin cells
ple response is shown here.
1 chromosome
DNA
gene on a
chromosome
Teacher’s Guide
D-119