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The Work of Gregor Mendel Essential Questions Where does an organism get its unique characteristics? How are different forms of a gene distributed to offspring? Who was Gregor Mendel? Monk born in 1822 in Czech Republic. At age 21, entered a monastery Performed scientific experiments in the garden. Discovered the principles of heredity(genetics), the passing of traits from parents to offspring. Mendel’s Model System Pea Plant – Pisum sativum Capable of Self-fertilization (pollination) Cross-fertilization (pollination) Reproduces quickly Yields high # of offspring Studied distinctive phenotypes Flower color Stem length Pod shape of color Seed shape or color Self vs. Cross Pollination Self vs. Cross Fertilization Pea plant Cross Pollination Process Stigma Carpel Pollen Anthers a. This flower has been sectioned to show the location of its anthers (male) and of the carpel with its attached stigma (female). Pollen grains form in the anthers. Egg cells develop, fertilization takes place, and seeds mature inside the carpel. b. Pollen from one plant is brushed onto the stigma of a second plant. The anthers have been cut from the second plant so that it cannot selffertilize. c. The cross-fertilized plant produces seeds, which may be scored for seed traits, such as smooth or wrinkled shape, or may be grown into plants for scoring of adult traits, such as flower color. d. The adult pea plant (F1 generation) Fig. 14-2a TECHNIQUE 1 2 Anther (male parts) Parental generation (P) Stigma(female) 3 4 RESULTS 5 First Filial generation offspring (F1) Unraveling the Mystery Characteristics Mendel studied only one pea trait at a time. (Monohybrid crosses) Trait: Characteristic that has different forms in a population. (Ex. Seed color) Mendel cross pollinated to learn more about how traits are inherited. Unraveling the Mystery Mendel used plants that were true breeding (homozygous) for each trait he was studying. DO NOW: What did Mendel do to guarantee the plants he wanted to cross pollinate did not self pollinate first? Objectives: 1. Describe Mendel’s studies and conclusions about inheritance. 2. Explain the relationship between traits and heredity. 3. Differentiate between dominant and recessive traits. 4. Explain how genes and alleles are related to genotype and phenotype. Mendel’s First Experiments Studied seven different characteristics. Noticed one trait was always present in the 1st generation, and the other trait seemed to disappear. Dominant trait: Trait that appeared Recessive trait: Trait that seemed to fade into the background. Mendel’s Second Experiments Mendel allowed the 1stgeneration (F1) plants to selfpollinate (heterozygous cross). Recessive trait reappeared in 2nd generation plants (F2). Hybrids: Offspring of crosses b/w parents with different traits. Monohybrid cross One allele different: Flower color P=Purple (dominant) p=white (recessive) Parental generation PP x pp Possible parental gametes: ♀= P ♂=p Mendel’s Second Experiments Results of the F2 generation: Recessive trait did not show up as often as the dominant trait. Mendel figured out the ratio of dominant traits to recessive traits Mendel’s Second Experiments In all cases the ratio was about 3:1 dominant : recessive. Mendel’s Summary Genes: Characteristics determined by factors that are passed from the parental generation. Alleles: 2 different forms of a gene. Example: Trait = Seed shape Allele 1 = Round seed; Allele 2 = Wrinkled seed Gene = Round seed is inherited because it is a dominant allele (masks recessive allele) Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers Explaining the F1 Cross: Segregation • Dominant alleles mask recessive allele in the F1 generation but reappear in the F2 generation. • Conclusion: Alleles separate during the formation of sex cells (gametes) and recombine during fertilization (zygote) Explanation of Segregation • F1 plant • Inherited an allele for tallness from its tall parent and an allele for shortness from its short parent • Result: Tt • All plants tall Explanation of Segregation • When each parent, or F1 adult, produces gametes (sperm or egg cell), the alleles for each gene segregate from one another, so that each gamete carries only one allele for each gene. Explanation of Segregation • Each F1 plant produced two kinds of gametes: • Those with the allele for tallness (T) and those with the allele for shortness (t). Explanation of Segregation • The F2 plant inherits one of each gamete in different combinations • Combination possibilities: • TT = Tall • Tt = Tall • tt = Short Explaining Mendel’s Results (Summary) Proposed theory of Particulate inheritance Hereditary determinants: Do not blend together Do not become modified through use Maintain their integrity Exist in distinct discrete units (genes) Each individual has two versions (alleles) of each gene Terms Genetics Scientific study of hereditary Gene Hereditary determinant Alleles Two different versions of the same genes Genotypes All of the alleles found in an individual Phenotype The physical traits which are observed DO NOW: If you flip a coin what are the chances it will land on heads? Tails? Suppose that you flipped the coin and got heads. What are the chances that you will get heads again? Objectives: 1. Explain how geneticists use the principles of probability to make Punnett squares. 2. Describe the principle of independent assortment. 3. Solve genetic word problems using a Punnett square. Applying Mendel’s Principles Essential Questions How can we use probability to predict traits? How do alleles segregate when more than one gene is involved? What did Mendel contribute to our understanding of genetics? Phenotype vs. Genotype Phenotype : Organism’s appearance (purple flowers). Genes affect the phenotype. Genotype: Combination of inherited alleles together. Homozygous: Genotype that has 2 dominant OR recessive alleles (PP or pp). Heterozygous: Genotype that has one recessive and one dominant (Pp). What are the Chances? Gene: 2 Alleles Probability: Mathematical chance that something will happen. Determines phenotypic ratios in offspring. Calculating Probability What are the Chances? Genotype Probability Each offspring of a Pp X Pp cross has a 50% chance of receiving either a P allele or p allelle from the parent. So, the probability of inheriting two p alleles is 1/2 X 1/2, which equals 1/4, or 25%. Probability in Mendel’s Crosses Probability of PP zygote = ½ × ½ = ¼ Probability of pp zygote = ½ × ½ = ¼ Punnett Squares organize all the possible genotype combinations of offspring from particular parents. What are the genotypes? Phenotypes? 1. What is the genotype of the offspring represented in the upper left-hand box of the Punnett square? A Homozygous dominant B Homozygous recessive C Heterozygous 2. What is the genotype of the offspring represented in the lower right-hand box of the Punnett square? A RR B Rr C rr D rrr 3. What is the ratio of Rr (round seeds) to rr (wrinkled seeds) in the offspring? A 1:3 B 2:2 C 3:1 D 4:0 Two Factor Crosses Law of Independent Assortment: Alleles of genes that govern two different characters segregate independently during formation of gametes. RR, Rr= Round; rr=wrinkled YY, Yy = Yellow; yy= green Cross: Rr Yy x Rr Yy Gametes (pollen) Gametes (eggs) Phenotypic ratio: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green Monohybrid vs. Dihybrid Test cross T T TT x tt t t Tt Tt Tt Tt Tt x tt t t T t Tt tt Tt tt A Summary of Mendel’s Principles Early 1900s, geneticist Thomas Hunt Morgan used fruit flies for genetic experiments. Ideal because it produces plenty of offspring. Conclusions: Mendel’s principles applied to most traits in most organisms. TWO-FACTOR CROSS In pea plants, green pods (G) are dominant over yellow pods (g), and smooth pods (N) are dominant over constricted pods (n). A plant heterozygous for both traits (GgNn) is crossed with a plant that has yellow constricted pods (ggnn). What are the probable genotypic and phenotypic ratios for this cross? GgNn x ggnn GN Gn gN gn gn GgNn Ggnn ggNn ggnn gn GgNn Ggnn ggNn ggnn gn GgNn Ggnn ggNn ggnn gn GgNn Ggnn ggNn ggnn Solution! Genotypic ratio 4 GgNn : 4 Ggnn : 4ggNn : 4 ggnn = 4:4:4:4 = 1:1:1:1 Phenotypic ratio 4 green smooth : 4 green constricted : 4 yellow smooth : 4 yellow constricted = 4:4:4:4=1:1:1:1 DO NOW: Construct a Punnett square that demonstrates the cross between 2 pea plants that are heterozygous for purple color and seed shape (round seeds are dominant over wrinkled seeds). Objectives: 1. Describe other patterns of inheritance that are exceptions to Mendelian genetics. 2. Explain the relationship between genes and the environment. Chapter 11.3: Essential Questions What are some exceptions to Mendel’s principles? Does the environment have a role in how genes determine traits? Other Patterns of Inheritance Mendel worked with a genetically simple system Most traits are controlled by a single gene Each gene has only 2 alleles, 1 of which is completely dominant to the other There are many exceptions to simple Mendelian genetics. Incomplete Dominance Heterozygote has an intermediate phenotype between that of either homozygote (one trait is not completely dominant over another). When a red and white snapdragon flower breed it makes offspring that are all pink. Example in humans: Wavy hair Incomplete dominance X true-breeding true-breeding P red flowers white flowers F1 100% pink flowers 100% generation (hybrids) self-pollinate F2 generation 25% red 50% pink 2005-2006 25% white 1:2:1 Incomplete Dominance in Human Traits Normally, red blood cells are round and disk-shaped. With sickle cell anemia the red blood cells are sickle-shaped. Incomplete Dominance in Human Traits Sickle-cell disease Homozygote recessive has sickle-cell disease Heterozygote has milder sickle-cell trait Codominance Codominance: Organisms have two different alleles of a gene and both phenotypes show at the same time. Codominance Both alleles for the trait are dominant, resulting in offspring with both phenotypes. Example: Crossing a homozygous white cow with a homozygous red cow produces a roan cow with a coat of red AND white spots. Codominance in Humans ABO blood groups 3 alleles IA, IB, i Both IA & IB are dominant to i allele IA & IB alleles are codominant to each other Determines which antigens are produced on the red blood cells. Human ABO Blood Group Immune system produces antibodies against antigens not found on its own red blood cells Blood donation 2005-2006 Human Blood Types Greatest percentage in U.S? Total positive RH factor? Negative? Percentage that can be used for the most transfusions? The least? Could a person with O+ have 2 parents that are O-? Could that person have a daughter with AB+ blood? Multiple Alleles This term describes traits for which there are more than 2 alleles. Ex: A gene for animal fur color has 4 different alleles Polygenic Traits Polygenic Traits: Inherited traits that are determined by more than one gene. Examples: Eye color in humans have at least 3 different genes with multiple alleles. Human Height Polygenic inheritance Phenotypes on a continuum: Environment will also influence polygenic traits. Human traits skin color height weight eye color intelligence behaviors Environmental Factors Genes aren’t the only influences on traits. Genes provide a plan but that plan also depends on the environment. Ex: Height and skin tone Environmental factors In certain reptiles, sex is determined by temperature. During development in the egg, higher temperatures favor the production of males. Nature vs. Nurture Phenotype is controlled by both environment & genes Color of Hydrangea flowers is 2005-2006 influenced by soil pH Human skin color is influenced by both genetics & environmental conditions Coat color in rabbits is sometimes influenced by temperature also Coat color in arctic fox influenced by heat sensitive alleles Objectives: 1. Contrast the number of chromosomes in body cells and in gametes. 2. Summarize the events of meiosis. 3. Contrast meiosis and mitosis. 4. Describe how alleles from different genes can be inherited together. 5. Explain how chromosomes determine sex. Chapter 11.4: Essential Questions How many sets of genes are found in most adult organisms? What events occur during each phase of meiosis? How is meiosis different from mitosis? How can 2 alleles from different genes be inherited together? Asexual Reproduction Asexual reproduction Only one parent cell is needed. Structures inside the cell are copied, and then the parent cell divides, making two exact copies This type of cell reproduction is called mitosis. Most of the cells in your body (body cells) reproduce this way. Cell Cycle Interphase G1 S G2 Mitosis Prophase Metaphase Anaphase Telophase Mitosis Sexual Reproduction In sexual reproduction, two parent cells (gametes) join together to form offspring that are different from both parents. Sexual Reproduction Genes and Chromosomes Walter Sutton studied meiosis in sperm cells in grasshoppers. Using his observations and his knowledge of Mendel’s work, Sutton proposed that: Genes are located on chromosomes. Chromosomes Chromosomes: Structure consisting of DNA that transmit genetic information to each subsequent generation Homologous chromosomes: Chromosomes that carry the same sets of genes. One chromosome from the father and one from the mother. Homologous Chromosomes Sexual Reproduction Body Cells Human body cells have 46 chromosomes in 23 pairs. Human body cells are referred to as somatic cells. • Examples: Liver cells, heart cells Gametes (Sex cells) Gametes have only one of the chromosomes from the pair (total of 23 chromosomes per cell) Gametes are made during meiosis. • Examples: Sperm and Egg Meiosis that produces sex cells Meiosis is a copying process (sperm and egg) with half the usual number of chromosomes. Why? When the sperm (23 chromosomes) combines with the egg (23 chromosomes) it produces a cell with 46 chromosomes. Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues. Homologue Homologue Autosomes (The Autosomes code for most of the offspring’s traits) In Humans the “Autosomes” are sets 1 - 22 Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd set XX chromosome - female XY chromosome - male Sex Chromosomes “Sex Chromosomes” …….the 23rd set This person has 2 “X” chromosomes… and is a female. 23 Steps of Meiosis Meiosis consists of 2 distinct divisions: Meiosis 1 & Meiosis 2 The result at the end will be 4 haploid cells with half the number of chromosomes in the body cells. Meiosis Meiosis: Prophase 1 Maternal + paternal chromosomes separate & replicate. Duplicated homologous chromosomes pair with one another. Prophase 1: Tetrads When homologous chromosomes pair up they form a structure called a tetrad, which contains 4 chromatids (chromosomes that have been duplicated). Prophase 1: Crossing Over undergo As tetrads form, they crossing-over. Genes from one chromatid are exchanged with genes from a chromatid of the homologous chromosome it matches with. Crossing-over produces new combinations of alleles. Meiosis Metaphase I Homologous pairs align at the metaphase plate (middle) Nuclear membrane dissolves and the homologous chromosomes attach to spindle fibers Meiosis Anaphase I Spindle fibers pull each homologous chromosome pair toward opposite ends of the cell Meiosis: Metaphase 1 and Anaphase 1 Metaphase 1: Homologous pairs align independently at the metaphase plate. Anaphase 1: Homologous chromosomes separate and move toward the poles. Meiosis: Telophase 1 and Cytokinesis Cell divides into daughter cells. Pairs of homologous chromosomes are separated randomly. Sets are shuffled and sorted to 2 separate daughter cells. Meiosis 2: Prophase 2 Cells have one homologous pair from each tetrad. (Tetrads were separated during Meiosis 1) Meiosis 2: Metaphase 2 Chromosome pairs align at the metaphase plate. Spindle fibers attach to the centromeres of the chromosomes. Meiosis 2: Anaphase 2 Sister chromatids separate and become daughter chromosomes. Meiosis 2: Telophase 2 and Cytokinesis Spindle disappears, nuclei form, and cytokinesis takes place. With the formation of 4 cells, meiosis is over. Each gamete cell carries half the number of chromosomes of body cells. Meiosis Comparing Meiosis and Mitosis Mitosis Meiosis When the duplicated chromosomes separate each daughter cell receives one complete set of chromosomes. Does not change the chromosome number of the original cell. Results in the production of two genetically identical diploid cells. Asexual reproduction One major division Homologous chromosome pairs line up and then move to separate daughter cells. Reduces the chromosome number by half. Produces 4 genetically different haploid cells. Sexual reproduction Two major divisions Meiosis vs. Mitosis Objectives Contrast the number of chromosomes in body cells and in gametes. 1. Summarize the events of meiosis. 2. Contrast meiosis and mitosis. 3. Describe how alleles from different genes can be inherited together. 4. Explain how chromosomes determine sex. Gene Linkage Thomas Hunt Morgan’s research on fruit flies led him to the principle of gene linkage. After identifying more than 50 Drosophila (fruit fly) genes, Morgan discovered that many of them appeared to be “linked” together in ways that seemed to violate the principle of independent assortment. Gene Linkage Linkage is defined genetically as the failure of two genes to assort independently. Linkage occurs when two genes are close to each other on the same chromosome. However, just because 2 genes are on the same chromosome doesn’t mean they are linked. Genes far apart on the same chromosome assort independently during crossing over: they are not linked. Gene Linkage Gene Linkage Morgan’s findings led to two remarkable conclusions: First, each chromosome is actually a group of linked genes. Second, it is the chromosomes that assort independently, not individual genes. Alleles of different genes tend to be inherited together when those genes are located on the same chromosome. Gene Mapping In 1911, Columbia University student Alfred Sturtevant wondered if the frequency of crossing-over between genes during meiosis might be a clue to the genes’ locations. Sturtevant reasoned that the farther apart two genes were on a chromosome, the more likely it would be that a crossover event would occur between them. If two genes are close together, then crossovers between them should be rare. If two genes are far apart, then crossovers between them should be more common. Gene Mapping By this reasoning, he could use the frequency of crossing-over between genes to determine their distances from each other. Sturtevant gathered lab data and presented a gene map showing the relative locations of each known gene on one of the Drosophila chromosomes. Sturtevant’s method has been used to construct gene maps ever since this discovery.