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The Daily Variety
Overview
Students will examine the differences between wild-type and mutant lines of the
same species of plant as a basis for understanding what an allele is and how variation in a
population occurs.
Introduction
Most organisms are diploid, having two sets of chromosomes, and therefore two
copies (called alleles) of each gene. However, some organisms can be haploid, triploid,
or tetraploid (having one, three, or four sets of chromosomes respectively). Within any
single organism, there may be variation between the two (or more) alleles for each gene.
This variation is introduced either through mutation of one of the alleles or as a result of
sexual reproduction. During sexual reproduction, offspring inherit alleles from both
parents and these alleles might be different.
A phenotype literally means “the form that is shown”; it is the outward, physical
appearance of a particular trait. In contrast, a genotype is the specific allelic combination
for a certain gene or set of genes. In classical genetics, phenotype can be observed
directly, but genotype can has to be inferred from phenotypes of the individual or the
individual’s parents and/or progeny.
It is important to be able to talk about how a population varies genetically, and in
order to do so, scientists must be able to describe what is “normal” in a population.
Therefore, scientists must decide what the normal trait or allele is. This “normal” allele is
called wild-type. Wild-type is somewhat arbitrarily designated and is usually a reflection
of one of the major phenotypes of in a population. Any allele that is not wild type can be
thought of as a “mutant”, or alternative allele. For Wisconsin Fast Plants, scientists have
decided what wild-type height and color look like and have designated a wild-type plant.
Alternative allele forms are usually the result of mutations in the DNA that may
have arisen a long time ago. An allele codes for the production of amino acids that string
together to form proteins. Thus differences in the nucleotide sequences of alleles result in
the production of slightly different strings of amino acids and therefore variant forms of
the proteins. These proteins code for the development of the anatomical and
physiological characteristics of the organism, which are also responsible for determining
aspects of the organism, for instance physical appearance and behavior.
With more than one allele form are around, there is usually a pecking order on the
effect of these alleles. There are alleles which are dominant and alleles that are
recessive. A dominant allele masks the expression of a recessive allele. Therefore, the
dominant allele will determine the phenotype of a heterozygous organism. There are
some exceptions to this rule. When an organism has codominant alleles, both traits are
shown. And when an organism has alleles that are incompletely dominant, there is a
blending of the phenotypes.
Motivation
Wouldn’t the world be a different place if we all looked exactly the same?
Luckily for us we don’t all look the same. How do we describe difference and how can
we make predictions about what our children might look like? The answer lies in
genetics. In this lab, we will look at how variation occurs in a population using the
vocabulary of genetics.
Objectives
Upon completion of this lab, students should be able to
1. Define and describe allele and wild type.
2. Demonstrate the difference between wild-type and a variety of mutants using pictures
and descriptive vocabulary.
3. Discuss why different alleles of a gene are necessary for genetic variation in a
population.
4. Discuss how a mutation in the DNA sequence of an allele leads to a different
phenotype.
5. Identify that some alleles show dominance to other alleles.
Materials
• For this lab you may have the groups plant and grow their own Fast Plants, or you can
plant them ahead of time. For growing instructions, refer to your handout on growing
Fast Plants, or see http://www.fastplants.org/instructions/growing_instructions.html
• Several different mutant types of Fast Plant. Carolina Biological Supply has a variety
of mutations or lines available for Fast Plants, including Purple Stem, Hairy Stem,
Yellow Leaves, Dwarf (rosette height), and variegated leaves.
• Wild type “Standard” Fast Plants
• Pencil and paper for sketching
Associated California Biology Standards
2c. Students know how random chromosome segregation explains the probability that a
particular allele will be in a gamete
2d. Students know new combinations of alleles may be generated in a zygote through the
fusion of male and female gametes (fertilization).
4c. Students know how mutations in the DNA sequence of a gene may or may not affect
the expression of the gene or the sequence of amino acids in an encoded protein.
7c. Students know new mutations are constantly being generated in a gene pool.
Procedure
1. Have students sketch each of the different types of plants available.
2. After sketching the plants, have students write a descriptive explanation of the
characteristics of each plant’s phenotype, as if you where making a police mug shot.
3. Next to each sketch have students write a description that would allow them to
distinguish this plant from all the others. Identify which of the plants is the wild-type
plant. Have students write how each one of the mutants differs from the wild-type
phenotype.
4. After students have described the phenotype of the plants and studied the wild-type
plant, students should make a list of all the traits for which they have seen variation.
For instance, one of the traits might be “stem color” if students have observed green
stems and pink stems. When students have made up their list of traits they should
write down some possible allele names for each of these variations.
Evaluation
The following questions are listed under the Analysis section of the student handout and
may be used as part of a report, class discussion, or assessment
1. How are your plants phenotypically different? The same?
2. Which different traits showed variation in your population of plants?
3. Assuming that each trait is controlled by a single gene locus, how many different
alleles where represented in all the plants you studied?
4. Describe how variation of a trait might lead to increased survival in the wild.
5. What is dominance referring to when we talk about alleles?
6. How would you test to see if a particular allele of a trait was dominant over other
alleles?
Extension Activities
1. Some human traits are caused by a single gene. Recall that for each gene every person
has 2 alleles, one allele is inherited from their mother and the second allele is
inherited from their father. Attached ear lobes (recessive), tongue rolling (dominant),
widows peak (dominant), freckles present (dominant), red-green color blindness
(recessive), hitchhikers thumb (recessive), having dimples (dominant) and mid-digital
hair on your fingers (dominant) are all traits controlled by alleles of a single gene.
Record the number of people in your class with the following traits: red/green
colorblindness, a widow's peak, attached ear lobes, can roll tongue, have a
hitchhiker's thumb, have dimples, have freckles, have mid-digit hair on their fingers.
Once you have a tally, ask the class which of each of the respective traits is most
common (e.g. freckles or no freckles) and determine what the average student might
look like in their class. Once you’ve established what this average student, whom we
will call wild-type looks like, then have each student write how they differ from that
wild type average student.
Test Preparation
1. The expression of both alleles for a trait in a hybrid individual is
(A) pleiotropy
(B) epistasis
(C) codominance
(D) complementation
(E) incomplete dominance
2. A diploid cell has three pairs of homologous chromosomes: AaBbCc. How many
different gametes can this cell produce?
(A) 4
(B) 8
(C) 16
(D) 32
(E) 64
3. In guinea pigs, black coat color (B) is dominant. What is the most likely genotype of
the parents if they have 10 offspring, 5 of which are white?
(A) Bb x bb
(B) Bb x Bb
(C) BB x bb
(D) Bb x bb
(E) BB x BB
Name:___________________
Student Handout: The Daily Variety
Procedure
1. Sketch each of your plants.
2. After sketching your plants, write a descriptive explanation of the characteristics of
each plant’s phenotype, as if you where making a police mug shot.
3. Next to each sketch write a description that would allow you to distinguish this plant
from all the others. Identify which of your plants is the wild-type plant. How does
each one of your mutants differ from the wild-type phenotype?
4. After you have described the phenotype of the plants and studied the wild-type plant,
make a list of all the traits for which you have seen variation. For instance, one of the
traits might be “stem color” if you have observed green stems and pink stems. After
you have made up your list of traits, write down some possible allele names for each
of these variations.
Analysis
Answer the following questions on a separate piece of paper:
1. How are your plants phenotypically different? The same?
2. Which different traits showed variation in your population of plants?
3. Assuming that each trait is controlled by a single gene locus, how many different
alleles where represented in all the plants you studied?
4. Describe how variation of a trait might lead to increased survival in the wild.
5. What is dominance referring to when we talk about alleles?
6. How would you test to see if a particular allele of a trait was dominant over other
alleles?