The Huntington Library, Art Collections, and Botanical Gardens

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1 The Huntington Library, Art Collections, and Botanical Gardens 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.

2 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 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.

3 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.

4 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) 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

5 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?