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Mendel, First Geneticist
REA DI
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ACTIVIT Y OVERVIEW
SUMMARY
A reading describes Mendel’s experiments with pea plants. Students relate the rules discovered by Mendel in his analysis of pea plant crosses to their findings about critter
genes. The reading introduces the idea that basic concepts discovered in working with
one type of organism (for example, pea plants) can often be generalized to other
organisms or groups (such as humans). Students also consider the scientific approaches employed by Mendel in planning and analyzing his experiments.
KEY CONCEPTS AND PROCESS SKILLS
1.
Breeding experiments can provide information about the behavior of genes.
2.
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.
3.
Tracing the history of science demonstrates how individuals contributed to the
development of modern scientific ideas, and reveals important interactions
between science and society.
4.
An allele is one of the two or more forms of a gene present in a population.
5.
By virtue of his novel approach to biology, Mendel is considered by many as
the founder of the field of genetics.
6.
Scientists (such as Mendel) use many approaches to solve biological problems,
including careful observation, experimentation, and mathematical analysis.
Mathematical analysis applies to many areas of biology.
7.
Mendel studied a number of traits in order to develop his general principles of
heredity.
Teacher’s Guide
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Activity 60 • Mendel, First Geneticist
KEY VOCABULARY
allele
probability
characteristic
random
dominant
recessive
gene/genetics
sexual reproduction
offspring
trait
MATERIALS AND ADVANCE PREPARATION
For the teacher
Transparency 60.1, “Mendel’s Results”
12 transparent blue plastic disks
12 transparent orange plastic disks
*
1
overhead projector
*Not supplied in kit
TEACHING SUMMARY
Getting Started
1.
Introduce breeding and Mendel.
Doing the Activity
2.
Students read about and discuss Mendel’s life.
3.
The class discusses Mendel’s experiments and results.
Follow-Up
4.
The class discusses Mendel’s pattern of inheritance and models Mendelian
inheritance of the critter tail-color traits.
5.
Students respond to the Analysis Questions.
INTEGRATIONS
Social Studies and History of Science
This activity can be integrated with a study of other events of the time, including historical events in science (such as Darwin’s publication of Origin of Species, and the
events in Activity 37, “The History of the Germ Theory of Disease”). The importance
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Science and Life Issues
Mendel, First Geneticist • Activity 60
of plant and animal breeding efforts from prehistoric times to the present can also be
explored.
BACKGROUND INFORMATION
Gregor Mendel
Before Mendel, people’s ideas about heredity were similar to some of the hypotheses
students used in the critter simulation. People knew that pure-breeding lines of some
livestock and crops could be developed to reliably produce offspring with desired characteristics. They also knew that other traits were difficult to maintain by breeding,
but did not know that some traits are caused by many genes or are only partially determined by genes.
Although he had training in science, Mendel (like all scientists at the time) did not
know of the cellular basis of fertilization, the process of cell division, or the existence
of chromosomes. Yet Mendel was able to analyze the results of his experiments and
propose a theory of heredity that began the formal field of genetics. One of his most
important insights, which should be emphasized to students, was his realization that
he should use large numbers of crosses and offspring, and apply the mathematics of
probability to his results.
Mendel’s work received very little recognition until several years after his death when
three scientists working independently in 1900 simultaneously rediscovered the laws
of inheritance. They found that these laws explained the inheritance of many traits in
many species, although not all traits follow the simple one-gene patterns exhibited by
the pea traits reported by Mendel. (In fact, the inheritance of most traits is much more
complicated, involving in some cases many genes, multiple alleles for some genes, and
environmental factors in determining organisms’ characteristics.) Mendel’s theory of
inheritance is now the starting point for most genetics courses and is central to the
development and understanding of the theory of evolution by natural selection.
Mendel was educated in the natural sciences and worked as a teacher until he became
abbot of the monastery in 1868. He communicated with others interested in agriculture and science, and belonged to scientific organizations. He followed up his experiments with peas with investigations of other plants and with experiments on breeding and keeping bees. He was also known for his work in meteorology.
Students may be interested to know that despite Mendel’s sound preparation in science,
he twice failed the tests to obtain official teacher certification. One of his biographers
Teacher’s Guide
D-83
Activity 60 • Mendel, First Geneticist
has speculated that his failures were due to a nervous disposition in stressful situations; others have suggested that the testers disliked the unorthodox ideas about
earth and life sciences expressed by Mendel.
This reading gives you a chance to emphasize with your students an interesting
lesson of history: people with great and challenging ideas are sometimes overlooked during their lifetimes. Darwin suffered too, but far less than did Mendel.
Intellectual supporters flocked to Darwin’s side immediately after he published
Origin of Species. By the time of his death over 100 years ago, evolution by natural selection had achieved broad acceptance. Mendel’s work was an important
part of this. Developments in microscopy and studies of the cell around the turn
of the century allowed the scientific community a chance to understand the
deep significance and rigor of his results. These advances led to natural selection
getting the final cornerstone it needed—the particulate nature of inheritance.
Mendel didn’t live to see his work recognized, let alone placed at the foundation
of modern biology. But modern molecular biology and genetic engineering are
based on his groundbreaking research.
REFERENCES
Orel, Vitezslav. Gregor Mendel: The First Geneticist. New York: Oxford
University Press, 1996.
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Science and Life Issues
Mendel, First Geneticist • Activity 60
TEACHING SUGGESTIONS
GETTING STARTED
Introduce breeding and Mendel.
Initiate a discussion of breeding, using dogs, cats, or
other familiar organisms. For example, you might
ask, What is dog breeding? Why do people breed
dogs, cats, and other animals? Breeding refers to the
process of selecting two parent individuals to mate
(in the case of animals) or cross (in the case of plants)
in order to produce a desired outcome or improvement in the offspring. This might be done to produce
a dog or cat with a desired appearance or behavior, or
a cow that produces more milk. Introduce the idea
that people have been breeding animals and crops for
thousands of years and that breeding efforts have
been key to developing high-yield crops and transforming societies from hunter-gatherers to farmers.
Tell students that they will read about how an Austrian monk made a breakthrough in genetics by
breeding and carefully studying pea plants.
DOING THE ACTIVIT Y
2.
his environment that were favorable to his research.
The reading suggests that a thoughtful, methodical
approach characterized Mendel’s research. It also
suggests patience and a willingness to invest in a
long-term effort to obtain the data he needed. In
addition, students should note that Mendel, though
also a monk, had training in science and mathematics that helped to prepare him to form and
explore his ideas. His insights into genetics also suggest that he was creative and daring in his analysis
of his results.
3.
The class discusses Mendel’s experiments
and results.
After students read the first few paragraphs of Part
Two, again pause to discuss the Stopping to Think
questions. You may want to mention that modern
geneticists have also used organisms such as fruit
flies, with a 2-week generation time, and yeast,
with a generation time of hours, to study the
behavior of genes.
Explain that Mendel made sure he had purebred
plants. In other words, his original stock of greenseeded peas never produced yellow-seeded offspring, and vice versa. Students should recognize
Students read about and discuss Mendel’s
that this is equivalent to the critter-breeding exper-
life.
iment, in which the blue-tailed critter Skye was
This reading has two major purposes: 1) to give students an historical perspective on the development
of one of the major concepts in the biological sciences, and 2) to provide data from Mendel’s experiments breeding pea plants, which students can
compare to their own work with the critter model.
from an island containing only blue-tailed critters,
and the orange-tailed Poppy was from an island
containing only orange-tailed critters. The rest of
the critter scenario neatly parallel’s Mendel’s experiments as well. Emphasize the very large numbers
of crosses and the many seeds his crosses produced
for analysis. Without a large sample size, the 3:1
Plan to stop after students read Part One and discuss
ratio of offspring might not have been conclusive,
the first Stopping to Think question with the class.
or even apparent, as students may have observed
Discuss some of the characteristics of Mendel and
with their results from Activity 59, “Gene Combo.”
Teacher’s Guide
D-85
Activity 60 • Mendel, First Geneticist
Before students answer the questions, review how
mines which gene each Generation Two parent
to calculate ratios and clarify the relationship of
(such as Ocean and Lucy) donates to the offspring.
ratios to fractions. Figure 1 is intended to help illus-
Work through the four different combinations pos-
trate two different ways of expressing the number of
sible in the third generation (blue-blue, blue-
yellow- and green-seeded plants. A ratio expresses
orange, orange-blue, orange-orange). Discuss how
the relative proportion of one group to another, in
this works out to the 3:1 ratio of blue:orange tails in
this case 3 yellow:1 green or 1 green:3 yellow. Frac-
the story. In Activity 61, “Gene Squares,” students
tions express the relative proportion of one group to
will use Punnett squares to further investigate this
the total, as in 3/4 yellow, 1/4 green.
basic model of inheritance.
FOLLOW–UP
4.
Discuss the inferences Mendel made from his
results and how he used the actual numbers of dif-
The class discusses Mendel’s pattern of
ferent kinds of pea plants to explain and model how
inheritance and models Mendelian
the results arose. Use this example to help students
inheritance of the critter tail-color traits.
see the difference between the results or evidence of
Use Stopping to Think 3 to prompt a review of the
pattern of inheritance described by Mendel, commonly referred to as Mendelian inheritance. In this
pattern, a particular characteristic (such as flower
color) has two different versions, called traits (such
as purple vs. white). If an individual has alleles for
both versions of the trait, the dominant version
appears, while the recessive version is completely
masked. (Such an individual is called a hybrid.)
an experiment—the raw data and any calculations—and a conclusion or interpretation. Ask students to describe which aspects of the model are
orderly and which are random. Emphasize that the
process follows a set of rules Mendel was able to
describe. These rules are the basis of the assumptions of the coin-tossing model used for Activity 59.
The only random aspect is which of the two possible alleles is donated by each parent.
When two individuals who have alleles for both
Review the terms dominant and recessive. You may
traits produce offspring, approximately 1/4 of the
want to use the flower color cross (in Table 1 on
offspring show the recessive trait and approximate-
page D-36 in the Student Book) for a final review of
ly 3/4 show the dominant trait.
these concepts and of how to calculate ratios. This
At this point, have students consider Mendel’s
model as described on page D-37 in the Student
Book. Use the blue and orange transparent disks to
demonstrate the model on the overhead projector,
and if possible have student volunteers participate.
Place two blue disks for Skye and two orange disks
will help students begin answering Analysis Questions 1 and 2. Analysis Question 1 can be used to
assess the “Organizing Data” element of the
DESIGNING
AND
C O N D U C T I N G I N V E S T I G AT I O N S
(DCI) variable. A sample level 3 response is provided in Table 1.
for Poppy. Each Generation Two critter gets a blue
At this time, explain to students that the genetics
disk from Skye and an orange one from Poppy.
behind many human traits is far more complicated
Activity 59 modeled the random process that deter-
than Mendel’s model. Nevertheless, Mendel’s
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Science and Life Issues
Mendel, First Geneticist • Activity 60
model provides a foundation necessary for under-
that the two characteristics with the larger sam-
standing some of the more complicated inheritance
ple sizes—seed color and seed surface—have
patterns that will be presented in later activities.
ratios closer to the predicted value than the two
5.
characteristics with smaller sample sizes.)
Students respond to the Analysis Questions.
3.
1.
seeded to yellow-seeded offspring. Explain why a
TO ANALYSIS QUESTIONS
1:3 ratio of green-seeded plants to yellow-seeded
Based on Mendel’s results, what trait for
each pea characteristic is dominant? Make
plants is the same as a fraction of 1/4 green-seeded
DCI
od
plants.
a table of the dominant and recessive alternatives
A 1:3 ratio means that for every one green-seed-
for each characteristic in Table 1. Add an extra col-
ed plant there are (approximately) three yellow-
umn to your table; you will use it to record your
seeded plants. A ratio compares one group to
answers to Question 2a.
another, in this case green- to yellow-seeded
plants. A fraction compares one group to the
See Table 1 below.
2.
Look at Figure 1, which shows the ratio of green-
SUGGESTED ANSWERS
a.
total. In this case, the total number of plants is
Calculate to the hundredths place the ratio of
4. For every four plants total, there are about
dominant to recessive for each characteristic in the
three yellow-seeded plants (3/4) and one green-
third generation. Record the ratio for each charac-
seeded plant (1/4).
teristic in the table you prepared for Question 1.
4.
Mendel performed his experiments on more charac-
The ratios are shown in the fourth column of
teristics than the four shown in Figure 1. Why was
Table 1.
it important for him to look at more than one char-
b.
acteristic?
Why are the ratios not exactly 3:1?
They are not exactly 3:1 because they are actual
data from a small sample set, while the 3:1 ratio
is predicted for a large set of results, based on the
theoretical probability of each outcome. (Note
By looking at more than one characteristic,
Mendel determined that his observations were
generalizable and not just a fluke. Note that
similar results have been obtained with many
Table 1
Characteristic
Dominant
Trait
Recessive
Trait
Ratio
Dominant:Recessive
flower color
purple
white
705:224 = 3.15 : 1
seed color
yellow
green
6,022:2,001 = 3.01 : 1
seed surface
smooth
wrinkled
5,474:1,850 = 2.96 : 1
pod color
green
yellow
428:152 = 2.82 : 1
Teacher’s Guide
D-87
Activity 60 • Mendel, First Geneticist
5.
characteristics in many species of plants and
math is used to compare populations. For exam-
animals.
ple, they might say that mathematics helps us
Reflection: People often think of mathematics as
important to physics and chemistry, but not to life
science (biology). What is your opinion?
D-88
calculate average pulse rates. (Some more
sophisticated examples students will probably
not think of are studies of the rates of biochemical reactions and the interpretation of X-ray dif-
Challenge students to think of examples of
fraction images to determine the structures of
mathematics in life science. They are likely to
proteins and nucleic acids. Both of these exam-
think of measurement (of physiological indica-
ples involve very sophisticated mathematical
tors like pulse, blood pressure, and tempera-
analysis.) Some mathematicians devote their
ture), scale, and dosage. They may mention how
careers to studying biological problems.
Science and Life Issues
Science and Life Issues Transparency 60.1
purple x white
all purple
705 : 224
(purple:white)
Original Cross
(Generation One)
Generation Two
Offspring
Generation Three
Offspring
Flower Color
©2006 The Regents of the University of California
Seed Surface
6,022 : 2,001
(yellow:green)
all yellow
5,474 : 1,850
(smooth:wrinkled)
all smooth
green x yellow wrinkled x smooth
Seed Color
428 : 152
(green:yellow)
all green
green x yellow
Pod Color
Mendel’s Results
Teacher’s Guide
D-89