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55
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ACTIVIT Y OVERVIEW
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40
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Plants Have Genes, Too!
BO RA
T
SUMMARY
Students germinate seeds that are the offspring of plants bred from true-breeding
green and pale yellow strains of flowering tobacco. By predicting and then quantifying the colors of the offspring plants, students obtain genetic data for analysis.
KEY CONCEPTS AND PROCESS SKILLS
1. Genes are the units of information for inherited traits that parents transmit to
their offspring.
2.
All organisms have genes and traits.
3.
Organisms that reproduce rapidly are useful in studying genetics.
KEY VOCABULARY
gene, genetics
offspring
inherited
trait
MATERIALS AND ADVANCE PREPARATION
For the teacher
Transparency 55.1, “Parent and Grandparent Plants”
*
1
green overhead transparency pen
*
1
yellow overhead transparency pen
*
overhead projector
For each group of four students
10 Nicotiana seeds
1
clear plastic petri dish
1
piece of germinating paper
1
dropper
1
cup
Teacher’s Guide
D-17
Activity 55 • Plants Have Genes, Too!
*
*
access to sinks with running water
1
permanent-ink marker
*
masking tape
*Not supplied in kit
Plan to carefully monitor the use of the seeds, so enough will be left for Activity 64,
“Nature and Nurture.” Plan to have a space near a window where students can leave
the dishes with the germinating seeds for 10–14 days. Avoid placing the seeds in
south-facing windows during hot weather.
Prepare Transparency 55.1, “Parent and Grandparent Plants.” Either before or during
the activity use transparency pens to color the parent and grandparent plants as
shown in Figure 1 on page D-10 in the Student Book.
TEACHING SUMMARY
Getting Started
1.
Introduce the use of other organisms to study inherited traits.
Doing the Activity
2.
Explain the origin of the seeds used in the Procedure and have students set up
their seeds.
Follow–Up
3.
Discuss students’ predictions about what will happen when the seeds
germinate.
BACKGROUND INFORMATION
Genetics of the Flowering Tobacco Seeds
The seeds provided are from flowering tobacco plants, similar to those many people
have in their gardens. They are not the type of tobacco used to produce cigarettes and
other products. Plant crosses are performed by placing pollen from the anther of a
male plant directly onto the pistil of a female plant. The pollen migrates into the pistil and fuses with an egg cell within the ovary of the female plant, where it develops
into a seed.
D-18
Science and Life Issues
Plants Have Genes, Too! • Activity 55
The genetic analysis below is presented for your review only; the current activity and
several activities that follow allow students to discover Mendelian genetics for themselves. Mendelian traits such as the albino flowering tobacco trait considered in this
activity are determined by a single gene location. Different versions of the gene are
called alleles. In this case, there are alleles for green and for albino color. Each plant
has a pair of alleles for color, one allele from the maternal plant and one from the
paternal plant.
The parent and grandparent plants of the seeds provided are illustrated on Transparency 55.1. The grandparent plants are a normal green-flowering tobacco strain and
a purebreeding strain of albino mutants, which do not produce the chlorophyll
responsible for the green color of plants. The alleles of these plants can be written as
GG (green) and gg (albino). For the students, we refer to the albino plants as pale yellow, as that is how they are likely to describe the color.
The parents of the seeds students will germinate are the offspring of the cross
between the GG and gg plants. They all have the allele combination Gg (they get one
G allele from the green grandparent and one g allele from the albino grandparent.)
These parents all appear green and totally normal, but they have a hidden recessive
allele and are thus called carriers of the pale yellow trait. (They can also be called
hybrids or heterozygotes. The terms carriers, homozygous, and heterozygous will be
introduced in later activities in the unit. At this time, avoid telling students the
genetic makeup of the seeds.)
When students germinate the seeds, the seedlings will demonstrate the Mendelian
segregation that occurred when the gametes (sex cells) were formed: Gg x Gg (carriers of albino trait) produces approximately 1/4 GG, 1/2 Gg, and 1/4 gg (a 1:2:1 genotype ratio). (The genotype is the genetic makeup of an organism.) Since GG and Gg
offspring are both the normal green color, this results in a color ratio of 3/4 green:1/4
albino (a 3:1 phenotype ratio). (The phenotype describes the physical characteristics
of an organism. Genotype and environmental conditions determine phenotype.)
These ratios describe the probability of different allelic combinations and appearances
in the offspring—they are rarely perfectly matched by real-world data.
Teacher’s Guide
D-19
Plants Have Genes, Too! • Activity 55
TEACHING SUGGESTIONS
GETTING STARTED
1.
Introduce the use of other organisms to
study inherited traits.
parent and one pale yellow parent. Explain that the
word offspring, instead of children, is used when referring to non-human organisms. Leave the transparency up as students begin the activity.
Do not try to guide students as they make their predictions of the colors of the offspring. Multiple
Have students read the Introduction on page D-9
hypotheses are desirable at this stage; encourage
in the Student Book or provide your own intro-
students to debate with one another. Students will
duction to the activity. Be sure they realize that
have a chance to modify their predictions as they
inherited traits are passed directly from the cells of
progress through the unit. In Activity 62, “Analyz-
the parent to the cells of the offspring. The genes
ing Genetic Data,” they will examine their results.
are the units of information. Emphasize this idea
of genes carrying information that specifies traits
of the offspring. Inherited traits may be contrasted with acquired traits, such as the highly developed muscles of body builders.
Have students follow the Procedure. Show them
where to place their dishes when they have finished setting up their seeds. Make sure that additional water is added one drop at a time. Excess
water encourages mold growth. However, even if mold
Ask students, What are some advantages of study-
begins to grow in the dishes, students can still usu-
ing heredity in plants and simple organisms such
ally detect the colors of the seedlings and analyze
as bacteria and yeast rather than in humans or
their data.
large animals? One significant reason is the short
FOLLOW–UP
generation time of these organisms. Other reasons
involve the practical issues of maintaining larger
organisms. And, of course, breeding experiments
cannot be done with humans for ethical reasons.
3.
Discuss students’ predictions about what
will happen when the seeds germinate.
Hold a discussion of students’ ideas. However, do
DOING THE ACTIVIT Y
2.
Explain the origin of the seeds used in the
Procedure and have students set up their
seeds.
not lead them toward a “correct” prediction. Some
students may predict that the pale yellow color will
“reappear” in some of the seedlings. You may wish
to suggest thinking about traits that can be followed
within their own biological families. Analysis Ques-
Display and discuss Transparency 55.1, “Parent and
tion 2 can be used as a formative assessment of stu-
Grandparent Plants.” The transparency displays the
dents’ ideas about genetics. Do not expect level 3
parent and grandparent plants that produced the
answers at this time, since students have not yet
seeds. Clarify the use of the word generation to dis-
explored dominant and recessive traits. A level 3
tinguish among the grandparents, the parents, and
answer is provided to indicate the kind of answer
the offspring seeds the students will plant. Each of
students would be expected to provide at the end of
the parents who produced the seeds had one green
the unit.
Teacher’s Guide
D-21
Activity 55 • Plants Have Genes, Too!
1.
SUGGESTED ANSWERS
heredity that you can use to guide your
TO ANALYSIS QUESTIONS
instruction. If you score them with the UC
Record in your science notebook your prediction for the color or colors of the plants that will
grow from the seeds. You may make more than one
possible prediction, but be sure to indicate which
Scoring Guide, do so only to get baseline information, not for grading purposes, as very few
students are likely to provide level 2 or 3
answers at this time.
you think is most likely to happen.
A complete and correct level 3 response follows:
Student answers will vary. Some will suggest all
I predict that there will be both yellow and
green, all pale yellow (albino), or a mixture. At
green offspring. My reason for this prediction
this time, accept all answers.
is that I think the parents must have gotten
some genes for yellow color from the grand-
2.
What are your reasons for each prediction you pro-
parents. These genes are hidden in the parents
posed for Question 1? Explain.
because the green color is dominant. The hid-
Urge students to express their reasons clearly,
den recessive gene in the parents can come out
but at this time, accept all answers.
again in their children if both parents have the
n Teacher’s Note: These questions can be used
recessive gene.
as a pre-assessment of students’ ideas about
D-22
Science and Life Issues
Parent and Grandparent Plants
Grandparents
©2006 The Regents of the University of California
Parents
Offspring
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Science and Life Issues Transparency 55.1
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D-23