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59
i n ut e s
e ss
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IN
ACTIVIT Y OVERVIEW
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1–2
50 -m
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40
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Gene Combo
V EST IGA
SUMMARY
Students use a coin-tossing simulation to model the pattern of inheritance exhibited
by many single-gene traits, including the critter tail-color characteristic. They relate
this model to the hypotheses they developed in Activity 58, “Creature Features.” The
activity provides them with a framework within which to interpret Mendel’s results
(presented in Activity 60, “Mendel, First Geneticist”) and their own seed-germination
results (analyzed in Activity 62, “Analyzing Genetic Data.”)
KEY CONCEPTS AND PROCESS SKILLS
1.
Hypotheses are based on evidence and can be revised in light of new evidence.
2.
Creating models is one way to understand and communicate scientific
information.
3.
Sexual reproduction involves the union of two sex cells and produces unique
individuals that show a combination of traits inherited from both parents.
4.
The ratio of dominant to recessive traits in the third generation of a purebred
cross provides an important clue about gene behavior. A statistically random
process determines which allele each parent transfers to the offspring.
KEY VOCABULARY
allele
inherit
dominant
model
fertilization
probability
gene
random
hypothesis
recessive
Teacher’s Guide
D-65
Activity 59 • Gene Combo
MATERIALS AND ADVANCE PREPARATION
For the teacher
*
1
Transparency 59.1, “The Coin-Tossing Model”
1
Transparency 59.2, “Gene Combo Totals”
1
overhead projector
For each pair of students
*
2
pennies
1
Student Sheet 59.1 “Gene Combo Results”
1
small cup (optional)
*Not supplied in kit
Gather pennies and prepare the transparencies and student sheets.
TEACHING SUMMARY
Getting Started
1.
Introduce the coin-tossing model.
2.
Introduce chance and probability.
Doing the Activity
3.
Students toss coins to model how genes are passed from parent to offspring.
4.
Collect and display the groups’ data and discuss Analysis Questions 1 and 2.
Follow-Up
5.
The class discusses the outcomes of the coin-tossing model.
Extension
Students post their results on the SALI page of the SEPUP website and compare their
results with results from other classes.
INTEGRATIONS
Mathematics
This activity, “Gene Combo,” deals with the topic of probability, which is often introduced in late elementary or early middle school. Probabilities can be expressed as
ratios, fractions, percents, or decimals, and thus involve these topics as well.
D-66
Science and Life Issues
Gene Combo • Activity 59
BACKGROUND INFORMATION
Probability
Provided that sample size is adequate, experimental coin tossing corresponds reliably
to theoretical predictions, and is therefore a very good model for Mendelian genetics.
However, this correspondence does not itself verify the hypotheses or principles of
Mendelian genetics. Actual data from breeding organisms are still to come in this unit,
in the reading about Mendel’s experiments that follows, and with the data on germinated seeds gathered by students in Activity 62, “Analyzing Genetic Data.”
Ratios vs. Fractions
Fractions are used to compare a part to the whole, while
ratios are commonly used to compare two parts of a
whole to each other. In the diagram shown here the
shaded part of the circle represents 3/4 of the whole
and the unshaded part represents 1/4 of the whole. The
ratio of the shaded part to the unshaded part can be
represented by 3/4 : 1/4. This can be simplified to 3:1.
Students often confuse ratios and fractions. Pie graphs such as this one can be used to
help them understand the difference between these two ways of expressing relationships among parts of a whole.
Conventions for Allele Notation
In this activity, students are instructed to notate the blue-tail allele as uppercase “T”
and the orange-tail allele as lowercase “t.” This is because it has already been established that blue tail is dominant to orange tail. However, it is also acceptable to use
“B” for blue and “b” for orange, since blue is the dominant trait, or any other notation that is clearly defined.
Genes, Characteristics, Traits, and Alleles
Genetics terminology can be confusing for students. A characteristic refers to one
observable or measurable feature of an organism. Students studied a number of
characteristics of humans in Activity 54, “Investigating Human Traits.” Each version
of a characteristic is called a trait. A characteristic can be caused by one gene or by
Teacher’s Guide
D-67
Activity 59 • Gene Combo
many genes. Each gene can exist in a number of different versions, or alleles.
The critter tail-color characteristic in this activity models the pattern typical
of classic Mendelian inheritance. A single gene codes for the tail-color characteristic. The gene has only two possible alleles and only two tail-color traits
(blue and orange) exist. The blue-tail trait is dominant; only one allele for blue
tail color is needed for the tail to be blue.
D-68
Science and Life Issues
Gene Combo • Activity 59
TEACHING SUGGESTIONS
GETTING STARTED
1.
Introduce the coin-tossing model.
Ask students, What conclusions were you able to
draw by the end of Activity 58, “Creature Features”? In particular, ask, How many genes for the
tail-color characteristic do you think each critter
has? Tell students that they will investigate a model
for the behavior of genes that assumes that each
equally likely to be chosen; there is no bias). Use this
example to operationally define the terms random
and probability. You may want to contrast this with
a non-random example of probability, such as an
upcoming sporting event. Ask if the winner will be
determined randomly. The answer is no; instead,
the outcome will depend at least partially on the
preparation, talent, and ultimate performance of
the rival teams. Contrast this with another situation, such as the selection of a winning raffle ticket,
which is a random process.
parent has two versions of the gene for tail color and
Encourage students to apply these concepts to the
that only one version from each parent is trans-
outcome of tossing a coin by asking, What are the
ferred to each offspring. Introduce the word allele,
chances that a coin toss will result in heads (vs.
which first appears in the Student Book in Activity
tails)? Students will probably say 50-50. The odds
60, “Mendel, First Geneticist.” An allele is a version
are equal for heads or tails because the process is
of a gene. In this activity, tail color is determined by
random. Tell students they will use the outcomes of
two different alleles; one provides information
coin tosses (heads or tails) to simulate the random
resulting in a blue tail and the other provides infor-
transfer of genes from parents to offspring. They
mation resulting in an orange tail. A coin-tossing
will then compare the results of the random simu-
simulation will be used to model a random process
lation to the results of the critter breeding to see if
for determining which of the two alleles a parent
this random model fits the results. They will assume
gives an offspring.
that Ocean and Lucy are one breeding pair chosen
n Teacher’s Note: In the coin-tossing model for this
from Generation Two; i.e., they are offspring of Skye
activity, the term version of a gene is used in place of
and Poppy and both have blue tails.
allele. As students develop an understanding of the
DOING THE ACTIVIT Y
need for a different term, you will introduce the
term allele.
2.
Introduce chance and probability.
Begin a discussion of chance, probability, and randomness by asking students what the chances are of
picking an ace of hearts from a deck of cards. Students should suggest 1 out of 52. This is correct as
long as the deck is a normal deck of cards and as
long as the choice of cards is random (each card is
3.
Students toss coins to model how genes are
passed from parent to offspring.
Review the model presented on page D-30 in the
Student Book and on Transparency 59.1. The
model is reproduced below. Each side of the coin
represents a single version of the gene, and each
parent contributes one version. The model assumes
that Ocean and Lucy each contain one copy of each
version of the gene, just as the coins contain one
Teacher’s Guide
D-69
Activity 59 • Gene Combo
side representing each version. This is justified by
their coin tosses are not completely random when
the fact that their parents were Skye and Poppy,
done by hand.
who each came from an island where all critters
had tails of the same color. Each member of Ocean
and Lucy’s generation must have an allele for blue
tail color, but also must have an allele for orange
tail color (since that is the only one Poppy can have
contributed); this is summarized in part (c) of the
model. Part (d) tells how to interpret results, based
upon the assumptions of the model.
Be sure to review the genetic shorthand of representing dominant and recessive traits as upper and
lower case letters. Note that any letter can be used,
as long as the upper and lower case of the same letter are used for the different forms of the gene.
Often the letter chosen is the first letter of either the
characteristic (tail color, T/t) or the dominant trait
for the characteristic (blue, B/b). Here, T/t is arbitrarily used.
The Coin-Tossing Model
a.
The outcome of a coin toss (heads or tails)
represents the one version of a tail-color gene
that is contained in the sex cell (sperm or
egg) contributed by a parent critter. Tails
represents the blue version and heads
represents the orange version.
b.
Depending on your student population, you may
want to provide students with some guidance on
how to construct the table required for Procedure
Step 6 before they begin the activity. If your students are proficient at constructing data tables, you
may wish to assess them on the “Organizing Data”
element of the D E S I G N I N G
AND
CONDUCTING
A future offspring critter receives a version of
I N VESTIG AT IO N S (DCI) variable. A level 3 response
the tail-color gene from each of its two
is shown in Table 1 below.
parents when fertilization occurs.
c.
Each side of the coin represents one of the
two versions of the tail-color gene carried by
each Generation Two critter, such as Ocean
and Lucy.
d.
Allow the students about 15 minutes to collect their
data and provide you with their results. Typical student results follow in Table 1. Note that the ratio of
the data calculated is 2.3:1; this simulation relies on
having a large sample size. Therefore, it is important
Blue tail color is dominant to orange tail
to stress the difference between the class results and
color. This means that if a critter has at least
those of pairs of students. Be prepared to ask students
one copy of the blue version of the gene, its
why a larger sample size is more scientifically valid.
tail is blue. A critter has an orange tail only if
it has no blue versions of the tail-color gene.
Table 1: “Gene Combo” Sample Results
Gene Combo
No. of Times
Tail Color
TT
5
blue
dure of the simulation on the overhead projector
Tt
6
blue
before handing out supplies. You may wish to make
tT
3
blue
tt
6
orange
Use a transparency copy of Student Sheet 59.1,
“Gene Combo Results,” to demonstrate the proce-
Totals
14 blue
small cups available to students if they suspect that
D-70
Science and Life Issues
6 orange
Gene Combo • Activity 59
4.
Collect and display the groups’ data and
you toss a coin to model gene behavior? The coin
discuss Analysis Questions 1 and 2.
toss models the fact that there is a 50-50 chance of
Collect the groups’ summary data and use them to
complete Transparency 59.2, “Gene Combo Totals.”
the parent passing a blue vs. an orange allele to the
offspring.
Students should copy the class totals into their sci-
Analysis Questions 1–3 are difficult for most stu-
ence notebooks. Then work with the students to
dents to complete without help. They are best done
perform the calculations required in Analysis Ques-
as a whole class discussion or in small groups fol-
tions 1 and 2. You may need to guide the class
lowed by class discussion. Discuss the ratios pro-
through the questions. If you wish them to have
duced by the model. Have students compare the dif-
further practice with this type of problem, you can
ferent groups’ results with the total results.
have each pair repeat the procedure for the data
Emphasize that the larger the group the closer
gathered in their groups.
results should be to the predicted 3:1 ratio.
Discuss the idea that with 20 coin tosses, as used in
Discuss the relationship between the colored-disk
this activity, you would expect to get heads-heads,
model used in the previous activity and the coin-
heads-tails, tails-heads, and tails-tails each about
tossing model. In this case, the coin toss simulates
1/4 of the time, or 5 times out of 20. However, note
a random process for determining which allele each
that heads-tails and tails-heads are essentially the
parent gives to its offspring. What were the assump-
same outcome for the critters (resulting in one each
tions built into this model? Each organism has
of the blue and orange alleles), so these two results
exactly two pieces of genetic information (alleles)
can be added. Also make the point that theoretical
for tail color. In addition, every sex cell produced by
predictions and actual outcomes are not identical,
a second-generation critter (such as Ocean or Lucy)
as their data clearly demonstrate. Most groups will
has a 50% or 1/2 chance of having a blue tail-color
not have a perfect 3:1 ratio of blue:orange tails. Stu-
gene, and a 50% or 1/2 chance of having an orange
dents should see that some groups get a higher ratio,
tail-color gene.
while others get a lower ratio, than the theoretical
ratio of 3:1. The whole class’s results will usually be
closer to 3:1 than many of the individual group
results, but still might be nearer to 2.5:1 or 3.5:1
than 3:1.
FOLLOW–UP
5.
Question 4 prompts students to relate the coin-tossing model to the Generation Three critter data from
Activity 58, “Creature Features.” Ask students to
compare the model in this activity to Hypotheses A,
B, and C. Which one was being modeled? Students
should suggest Hypothesis C, based on the equal
genetic contribution by each parent and the con-
The class discusses the outcomes of the
cept of a single dominant gene overwhelming a sin-
coin-tossing model.
gle recessive gene. The coin-tossing model adds the
Review the basic principles of probability and
heredity as illustrated by this activity. Ask, Why did
concept of a random mechanism for determining
which gene is contributed by each parent.
Teacher’s Guide
D-71
Activity 59 • Gene Combo
Tell students that Hypothesis C is the one that rep-
their prior misconceptions and construct an accurate
resents the modern understanding of heredity.
foundation on which they can build, both within
Exactly how it was discovered, and how that under-
this unit and in future courses. Activities 60 and 61
standing relates to sexual reproduction, will be dis-
provide evidence to support the hypothesis built into
cussed in the next few activities. The thought
the coin-tossing model; in Activity 61 students will
process the students have gone through in thinking
use Punnett squares to explore the random nature of
about alternate hypotheses is similar to the kind of
Mendelian inheritance in another way.
thinking scientists often engage in when trying to
solve a problem.
Question 5 provides an opportunity to confront the
common misconception that “the dominant trait is
the more common one.” It can be scored with the
■ Teacher’s Note : The terms heterozygous and
homozygous will be formally introduced in Activity 61.
Extension
U N D E R S TA N D I N G C O N C E P T S (UC) scoring guide
Students post their results on the SALI page of the
and used as a baseline assessment of students’
SEPUP website and compare their results with
understanding of the concept of dominant and
results from other classes.
recessive traits. Class discussion should uncover the
fact that dominance refers only to which trait is
found in an individual who has both types of alleles. A striking example of a human trait that is rare,
although dominant, is polydactyly, or extra digits
on the hand. When asked, most students will
assume that polydactyly is a recessive trait. The Marfan syndrome is another example of a trait that is
dominant, but rare. This is also an opportunity to
introduce the term recessive to describe a trait that
is observed only when two alleles for the trait are
present. A recessive trait is essentially masked, or
hidden, by a dominant trait.
At the end of the activity, review the coin-tossing
model and discuss new terms and their meaning in
the context of the critter tail-color model. Show students how new vocabulary terms can help them
express ideas by inserting the word allele in place of
versions of the tail-color gene on Transparency 59.1.
Genetics concepts can be difficult for students; the
development of these activities is intended to address
D-72
Science and Life Issues
This provides a larger sample size. Instructions for posting your classes’ results are
provided on the SALI page of the
SEPUP website.
Gene Combo • Activity 59
“About ____ of the offspring have blue tails, and
SUGGESTED ANSWERS
1.
TO ANALYSIS QUESTIONS
about ____ of the offspring have orange tails.”
What is the ratio of blue-tailed to orange-
About 3/4 of the offspring have blue tails, and
tailed critter pups? Use the class data to
about 1/4 of the offspring have orange tails.
answer this question:
a.
d.
Explain why the class obtained such a large ratio.
Divide the number of blue-tailed offspring by
For example, why isn’t the ratio of blue to orange
the number of orange-tailed offspring.
tails 1:1, that is, 1/2 blue and 1/2 orange?
ratio of tail colors =
number of blue-tailed offspring
number of orange-tailed offspring
Blue tails are much more likely because three
coin-toss combinations yield a blue tail, and
only one gives orange. This is because blue is
When 20 coin tosses from each of at least 12
dominant—only the blue-tail trait is observed
student pairs are combined, there is still a pos-
as long as there is at least one allele for blue tail
sibility that the full class’s data will be ambigu-
color present.
ous, with the calculated ratio falling around
2.5:1 or 3.5:1 instead of near 3:1. One way to
2.
You and your partner are about to toss two
coins 100 times. Predict about how many
improve results is to have the students toss
times the outcome would be:
more coins. Another option is to combine data
from other sections of the course. (Both of these
a.
methods increase sample size.)
heads-heads
about 25 times (1/4 probability on each toss)
If you have Internet access in the classroom, the
b.
best approach is to go to the SEPUP website and look up the data from multiple
heads-tails
about 25 times (1/4 probability on each toss)
classes. See the instructions at
c.
the SALI page to post your
results .
b.
tails-heads
about 25 times (1/4 probability on each toss)
Round this value to the nearest whole number.
d.
Then express it as a ratio by writing it like this:
about 25 times (1/4 probability on each toss)
: 1
(whole number)
tails-tails
3.
How sure are you that you will get exactly
the results you predicted for Question 3?
Students are likely to get ratios between 2:1 and
Explain your answer.
4:1.
You cannot be sure you will get exactly those
c.
Express this ratio as a pair of fractions, so that
you can use them to complete the following
sentence:
results. The answers for Question 2 are based
on probability and are the most likely results.
The real-world results are rarely exactly what is
Teacher’s Guide
D-73
Activity 59 • Gene Combo
predicted theoretically, due to random varia-
mates, most of the offspring will have blue tails?
tion in the set of observed coin tosses. Proba-
Why or why not?
bility allows us to predict how likely each result
is, but not the actual sets of results obtained.
baseline assessment of students’ understanding
(If students are having difficulty with this idea,
of the concept of a dominant trait. They will
you might compare this with similar situations.
have another chance to be assessed on a similar
For example, you would predict that a family of
question later in the unit. A sample level 3
four children would have two girls and two
answer follows:
boys. This is the most likely outcome, but certainly does not happen in all families with four
children!)
4.
This question provides an opportunity to get a
A dominant trait is a trait that you can always
observe if at least one allele for the trait is present. For example, the blue-tail trait is domi-
Look back at Activity 58, “Creature Features.” Do
nant and is observed even if an allele for anoth-
the results of the coin-tossing model match the Gen-
er trait (orange tail) is present. This does not
eration Three critter data? Explain.
mean that every time a pair of critters mates
The Generation Three critters were about 3/4
blue-tailed and 1/4 orange-tailed. So the results
of the Gene Combo model are consistent with
the Generation Three critter data.
most of the offspring will be blue-tailed. If both
parents have orange tails, for example, then all
their offspring will also have orange tails.
n Teacher’s Note: Students may use the word
gene interchangeably with allele at this time.
5.
Try to write your own definition of the phrase UC
dominant trait as it is used in genetics. Hint:
Does it mean that every time any pair of critters
D-74
Science and Life Issues
This is not a serious error at this point.
The Coin-Tossing Model
a.
The outcome of a coin toss (heads or tails) represents
the one version of a tail-color gene that is contained
in the sex cell (sperm or egg) contributed by a parent
critter. Tails represents the blue version and heads
represents the orange version.
b. A future offspring critter receives a version of the
tail-color gene from each of its two parents when
fertilization occurs.
c.
Each side of the coin represents one of the two
versions of the tail-color gene carried by each
Generation Two critter, such as Ocean and Lucy.
©2006 The Regents of the University of California
d. Blue tail color is dominant to orange tail color. This
means that if a critter has at least one copy of the blue
version of the gene, its tail is blue. A critter has an
orange tail only if it has no blue versions of the tailcolor gene.
Science and Life Issues Transparency 59.1
D-75
Gene Combo Totals
Coin Tossing Model Results
©2006 The Regents of the University of California
Student
Group
No. of
Blue Tails
No. of
Orange Tails
Totals
Science and Life Issues Transparency 59.2
D-77
Name
Date
Gene Combo Results
Offspring
Ocean’s
contribution
(T or t?)
Lucy’s
contribution
(T or t?)
Offspring’s
Offspring’s
genes
tail color
(TT, Tt, tT, or tt?) (blue or orange?)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
©2006 The Regents of the University of California
15
16
17
18
19
20
Science and Life Issues Student Sheet 59.1
D-79