chapter 15 outline
chapter 15 outline BIOL 1110
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This 5 page Class Notes was uploaded by Caitrín Hall on Tuesday January 26, 2016. The Class Notes belongs to BIOL 1110 at University of Connecticut taught by Bernard Goffinet in Summer 2015. Since its upload, it has received 12 views. For similar materials see Introduction to Botany in Biology at University of Connecticut.
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Date Created: 01/26/16
Genetics and the Laws of Inheritance 15.1 Gregor Mendel’s experiments with garden peas revealed the pattern of inheritance of genetic traits Before Mendel, people assumed hereditary material blended in offspring Hybrids – cross 2 individuals that differ in at least one trait look different than parents o Monohybrid cross – differ only in 1 trait o Hybrids crossed with each other some offspring resemble hybrids, others resemble original parental varieties o Traits of parental varieties were segregating – separating out in offspring of hybrid generation Advantages of using garden peas o True-breeding varieties that differ by single trait o Fast and easy growth o Reproduction can be controlled o Self-fertilization – gametes from single pea flower fuse to form healthy offspring; occurs normally in absence of pollination Experimenter sees traits present within single individual o Experimenter can remove anthers and transfer pollen between different varieties (cross-fertilization) Experimenter creates hybrids Mendel’s work o Outline: several generations of selfing, 2-directional crosses, hybrid selfing o 7 distinct traits: flower color, seed color, seed shape, pod color, pod shape, plant height, flower position o True-breeding plants produce F1 generations, which reveal dominant (expressed) and recessive (unexpressed) traits; express one 1 of 2 forms of trait observed in parents o Offspring produced through selfing of F1 = F2 – some recessive traits appear 75% of plants express dominant, 25% express recessive (3:1) o Presence of dominant factor determines appearance, or phenotype o Pattern of inheritance 1. Parents transmit info (factors = alleles)—not traits 2. Upon sexual reproduction, each offspring receives 2 factors (1 from each parent) 3. There are as many factors for a trait as there are forms of the trait 4. Factors don’t blend or alter each other 5. Expression of factor depends on nature of second factor (dominant versus recessive) o Punnett square – combinations of factors in the generation arranged in a box diagram o Mendel’s first law of heredity = Law of Segregation – factors for alternate forms of a trait segregate from each other in individuals with both factors o Testcross – cross-fertilization between plant showing dominant form and plant showing recessive form If any offspring display recessive trait, dominant parent must have had 1 dominant and 1 recessive trait 15.2 Mendel’s model in terms of genes, alleles, and chromosomes Traits are determined by genes; genes are found on chromosomes Alleles = different forms of the gene (length of nucleotides) Mendel hypothesized that every pea plant has 2 factors Homologous chromosomes – pairs of genes that have genes involved with the same traits o Gene coding for particular trait is located at the same site or locus o Homozygous – both members of the pair have the same allele of the gene (YY or yy) o Heterozygous – different “ “ Meiosis segregates alleles when homologous chromosomes separate each spore receives 1 chromosome with 1 copy of the allele (gametophytes & gametes have same allele 15.3 Variations on Mendelian genetics In incomplete dominance, heterozygote has an intermediate phenotype o Neither alleles are dominant or recessive o Phenotype is direct reflection of genotype o R allele codes for an enzyme in the formation of red pigment, while R’ allele makes defective enzyme RR’ genotype produces only enough pigment to make a pink flower In pleiotropy, a single gene affects several traits o 1 locus multiples traits o Profoundly alter phenotype In polygenic inheritance, several genes combine to affect single trait o Polygenes o Each individual gene in a group of polygenes makes a contribution to the phenotype of the organism o Traits controlled by polygenes have wide range of variability o More genes involved range of variability approaches continuous pattern phenotypes can no longer be separated from each other Environment can alter expression of phenotype 15.4 Genes and chromosomes More genes than there are chromosomes linked genes on the same chromosome Linked genes tend to be inherited together, but genes on the same chromosome do not always stick together (because of crossing over) Crossing over – homologous chromosomes come together and exchange pieces during the first prophase of meiosis; fairly common o Produces genetic recombination – formation of new combinations of alleles Genetic maps show the order and position of genes on chromosomes o Scientists used to use the frequency of crossing over to determine relative position of genes on chromosomes (farther apart genes cross over more) Dihybrid cross – cross between individuals or plant varieties that differ in 2 traits o Mendel’s Second Law of Heredity (Law of Independent Assortment) – ONLY IF traits are located on separate, nonhomologous chromosomes, the traits assort themselves independently Epistasis – when 2 genes interact in such a way that either gene is capable of stopping the phenotypic expression of the other o Dominant alleles code for functional enzymes, while recessive code for nonfunctional o Involves at least 2 genes; chromosomes can be homologous or not **Essay 15.2 is important** Chapter Wrap-up Examine and Discuss Self Test—Basic problems 1. In a population of wildflowers, blue flower color (B) is dominant to white flower color (b). Using a Punnett square, diagram a cross between two heterozygous, blue-flowered plants. From the results, answer the following questions. a. What fraction of the offspring will have blue flowers? _____. b. What fraction of the offspring will have white flowers? _____. c. What fraction of the blue flowering plants will be heterozygous like the parents? _____. d. Given a random blue-flowered plant, what would be the best way to determine its genotype? _____. 2. In garden peas, yellow seed color (Y) is dominant to green seed color (y), and round seed shape (R) is dominant to wrinkled seed shape (r). Using a Punnett square, diagram the following dihybrid cross: YyRr × YyRr. a. From your diagram, list the gametes formed _____. b. List the ratios of the phenotypes in the offspring _____. 3. In the same garden peas as in problem #2, diagram with a Punnett square the cross: YyRr × yyrr. a. List the gametes formed _____. b. List the ratio of the phenotypes in the offspring _____. Self Test—Advanced problems 4. In orchids, a true-breeding Cattleya with large purple flowersis crossed to another true-breeding orchid with small yellow flowers. Yellow flower color (Y) is dominant to purple color (y). Flower size, however, shows incomplete dominance. Thus LL produces a large flower, LL’ an intermediate flower, and L’L’ a small flower. All of the 1 hybrid orchids are YyLL’ and have yellow intermediate-sized flowers. The orchid grower wants to get a true-breeding large yellow-flowered orchid. a. What types of gametes do the F hybrid1 produce? _____. b. In a Punnett square show the genotypes and phenotypes that result from a cross of the F 1ybrid orchids. c. What fraction of the F 2 generation will be true-breeding for large yellow flowers? _____. 5. In squash, true-breeding plants for disk-shaped fruits are crossed with true- breeding plants with elongate fruit. The F pl1nts all have disk-shaped fruits. When the F p1ants are crossed, the F pla2ts have fruits in the ratio of 9 disk- shaped: 6 spherical: 1 elongate. a. What genetic mechanism of inheritance is most likely demonstrated by these squash plants? _____. b. Using the symbols A or a and B or b for the genes, indicate the genotypes for disk- shaped fruit, spherical fruit and elongate fruit in a Punnett square. Applying Concepts 1. Discuss the theory of blending inheritance, which was the prevailing theory in the 1800s when Gregor Mendel began working with peas. What is a major objection to the theory of blending inheritance? 2. Is Mendel’s second law, the Law of Independent Assortment, always accurate? Do traits always assort independently? 3. What is the phenomenon of incomplete dominance? Using red and white floral color in snapdragons, discuss the mechanism of incomplete dominance in molecular/enzymatic terms. 4. What is epistasis? How does it differ from polygenic inheritance?
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