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11–1 The Work of Gregor Mendel A. Gregor Mendel’s Peas B. Genes and Dominance C. Segregation

11–1 The Work of Gregor Mendel A. Gregor Mendel’s Peas B. Genes and Dominance C. Segregation 1. The F 1 Cross 2. Explaining the F 1 Cross 11–2 Probability and Punnett Squares A. Genetics and Probability B. Punnett Squares C. Probability and Segregation Probabilities Predict Averages.

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11–1 The Work of Gregor Mendel A. Gregor Mendel’s Peas B. Genes and Dominance C. Segregation

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  1. 11–1 The Work of Gregor Mendel A. Gregor Mendel’s Peas B. Genes and Dominance C. Segregation 1. The F1 Cross 2. Explaining the F1 Cross 11–2 Probability and Punnett Squares A. Genetics and Probability B. Punnett Squares C. Probability and Segregation Probabilities Predict Averages 11–3 Exploring Mendelian Genetics A. Independent Assortment 1. The Two-Factor Cross: F1 2. The Two-Factor Cross: F2 B. A Summary of Mendel’s Principles C. Beyond Dominant and Recessive Alleles 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Polygenic Traits D. Applying Mendel’s Principles E. Genetics and the Environment 11–5 Linkage and Gene Maps A. Gene Linkage Gene Maps Biology 1 Notes Chapter 11 (Introduction to Genetics)Prentice Hall; pages 262-274, 279-285

  2. Every living thing has a set of characteristics inherited from its parents. People didn’t always understand the process of inheritance, but they had an idea that certain characteristics are passed from generation to generation Examples: Native Americans developed more than 300 varieties of corn People of Peru developed potatoes by selecting and breeding wild starchy plants The domestic dog came from selectively breeding wolves It was not until the mid-nineteenth century that Gregor Mendel, an Austrian monk, carried out important studies of heredity Genetics- the scientific study of heredity Heredity- the passing on of characteristics from parents to offspring. Characteristics that are inherited are called traits. Mendel was the first person to succeed in predicting how traits are transferred from one generation to the next. Mendel chose to use the garden pea in his experiments for several reasons. Early Concept of Genetics

  3. Garden pea plants reproduce sexually, which means that they produce male and female sex cells, called gametes (egg and sperm). In a process called fertilization, the male gamete unites with the female gamete. The resulting fertilized cell, called a zygote, then develops into a seed. The transfer of pollen grains from a male to a female reproductive organ in a plant is called pollination. When he wanted to breed, or cross, one plant with another, Mendel opened the petals of a flower and removed the male organs. He then dusted the female organ with pollen from the plant he wished to cross it with. This process is called cross-pollination. By using this technique, Mendel could be sure of the parents in his cross. Pollen grains Remove male parts Transfer pollen Female part Male parts Cross-pollination Mendel chose his subject carefully

  4. He studied only one trait at a time to control variables, and he analyzed his data mathematically. The tall pea plants he worked with were from populations of plants that had been tall for many generations and had always produced tall offspring. Such plants are said to be true breeding for tallness. Likewise, the short plants he worked with were true breeding for shortness. True-breeding means that if they were allowed to self-pollinate, they would produce offspring identical to themselves. Mendel was a careful researcher

  5. Mendel studied seven different traits and each trait had two contrasting characteristics: Flower color Flower position Seed color Seed shape Pod shape Pod color Stem length A trait is a specific characteristic, such as seed color or plant height, that varies from one individual to another Mendel crossed plants with each of the seven contrasting characters and studied their offspring. He called each original pair of plants the P (parental) generation. He called the offspring the F1, or “first filial,” generation. Mendel’s Research

  6. Language of Genetics • Mendel called the observed trait dominant and the trait that disappeared recessive. • Always use the same letter for different forms of the same trait (alleles). • capital letter shows dominance, lowercase letter shows recessive • Example: Stem Height Tall allele (T) Short allele (t) • The way an organism looks and behaves is called its phenotype. • Example: tall or short • The allele combination (genetic make-up) an organism contains is known as its genotype. • Example: TT, Tt, or tt • An organism’s genotype can’t always be known by its phenotype. • An organism is homozygous for a trait if its two alleles for the trait are the same. • Example: TT or tt • An organism is heterozygous for a trait if its two alleles for the trait differ from each other. • Example: Tt

  7. In 1905, Reginald Punnett, an English biologist, devised a quick way of finding the expected genotypes possibilities (Punnett square) Punnett Squares are used to make phenotype and genotype predictions in order to predict/compare the genetic variations that will result from a cross A Punnett square for this cross is two boxes tall and two boxes wide because each parent can produce two kinds of gametes for this trait. The two kinds of gametes from one parent are listed on top of the square, and the two kinds of gametes from the other parent are listed on the left side. It doesn’t matter which set of gametes is on top and which is on the side. Each box is filled in with the gametes above and to the left side of that box. You can see that each box then contains two alleles—one possible genotype. After the genotypes have been determined, you can determine the phenotypes. Heterozygous tall parent T t T t T t T t Heterozygous tall parent Punnett Squares

  8. Parents (P): (phenotype) Purebred tall x Purebred short (genotype) TT x tt F1 all hybrids: Phenotype: tall Gentoype: Tt Mendel’s Experiment #1 T T t Tt Tt t Tt Tt

  9. Mendel completed similar experiments for all of the seven traits The offspring of crosses between parents with different traits are called hybrids. All of the offspring had the character of only one of the parents Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height Round Yellow Gray Smooth Green Axial Tall Wrinkled Green White Constricted Yellow Terminal Short Round Yellow Gray Smooth Green Axial Tall Mendel’s Experiment

  10. Biological inheritance is determined by factors that are passed from one generation to the next. factors that determine traits are genes each trait is controlled by one gene (genes are found on chromosomes) each trait has two different forms called alleles One form is always dominant over the other if the dominant allele is present, it will show up (expressed by a capital letter) the recessive allele will be exhibited only when the dominant allele is absent (expressed by a lower case letter) This is known as The Principle of Dominance In Mendel’s experiments, the allele for tall (T) plants was dominant and the allele for short (t) plants was recessive. Pea plants will be tall unless the allele for tallness is absent TT- Tall plant Tt- Tall plant tt- short plant Short plant Tall plant t t T T t T F1 All tall plants t T Conclusions from Experiment #1 P

  11. Mendel wondered if the recessive allele had disappeared completely. So, he crossed the F1 plants to produce the F2 (second filial) generation by letting the F1 plants self-pollinate. In every case, he found the recessive trait of the pair seemed to disappear in the F1 generation, only to reappear unchanged in one-fourth of the F2 plants. All of the F2 generation produced plants with the similar results The recessive allele reappeared. All of the traits (including height) showed a 3:1 ratio of 3 dominant to 1 recessive Seed shape Flower color Pod color Seed color Flower position Pod shape Plant height Dominant trait axial (side) purple yellow round green tall inflated Recessive trait terminal (tips) green short white yellow wrinkled constricted Going further …

  12. F1: (phenotype) (Hybrid) tall x (Hybrid) Tall (genotype) Tt x Tt F2: TTTtTt tt (tall) (tall) (tall) (short) Genotypic ratio: 1 TT: 2Tt: 1tt Phenotypic ratio: 3 tall: 1 short Mendel’s Experiment #2 T t T TT Tt t Tt tt

  13. Mendel concluded from his second experiment that alleles for shortness and tallness had segregated (separated) during the formation of gametes (reproductive cells) Each allele is located on different copies of a chromosome, one inherited from each parent. During meiosis the two alleles separate so that each gamete carries only a one copy of a gene During fertilization, these gametes can randomly pair to produce (up to) four various combinations of alleles. Homologous Chromosome 4 a A Terminal Axial Inflated D d Constricted T t Short Tall Conclusions from Experiment #2

  14. The Principle of Segregation states that every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles. Law of segregation Tt´Tt cross F1 Tall plant Tall plant T t t T F2 Tall Tall Tall Short t t t t T T T T 3 1 The Principle of Segregation

  15. Probability is the likelihood an event will occur The principles of probability can be used to predict the outcomes of genetic crosses A Punnett square can be used to determine the probability of the number of desired outcomes by the total number of possible outcomes Example: What is the probability of getting a plant that produces round seeds when two plants that are heterozygous (Rr) are crossed? Punnett square shows three plants with round seeds out of four total plants, so the probability is 3/4. It is important to remember that the results predicted by probability are more likely to be seen when there is a large number of offspring. r R RR Rr R Rr rr r Probability

  16. 1) Cross of homozygous dominant x homozygous recessive P _______ X _______ (genotype) Monohybrid Cross (one trait)G= green pea pods g= yellow pea pods GG gg G G F1 genotype: all Gg F1 phenotype: all Green g Gg Gg g Gg Gg

  17. 2) Cross two of F1 generation F1 ________ X _______ (genotype) F2 genotype & ratio: 1 GG: 2 Gg: 1 gg F2 phenotype & ratio: 3 green: 1 yellow Gg Gg G g G GG Gg g Gg gg

  18. Test Cross - used to distinguish between homozygous and heterozygous organisms • unknown organism is crossed with a homozygous recessive individual • if unknown is heterozygous, then 1/2 of offspring will be dominant and 1/2 will be recessive • if unknown is homozygous, then all offspring will be dominant

  19. Test Cross Example • Example: (unknown vs. homozygous recessive) • If unknown is heterozygous, then ½ the offspring will be dominant and ½ will be recessive • If unknown is homozygous, then all of the offspring will be dominant • A breeder has a yellow cat and wants to know if it is a purebred. To determine what the cat’s genotype is the breeder does a test cross with a brown cat (homozygous recessive) . All of the offspring turn out yellow. What is the genotype of the original yellow cat? Y Y Y y Yellow cat: either YY or Yy Brown cat: yy y Yy Yy y Yy yy Yy yy y Yy y Yy

  20. Mendel’s first experiments are called monohybrid crosses because mono means “one” and the two parent plants differed from each other by a single trait—height. Mendel performed another set of crosses in which he used peas that differed from each other in two traits rather than only one. Such a cross involving two different traits is called a dihybrid cross. Plant height Seed shape Seed color Tall Short yellow round green wrinkled Monohybrid vs. Dihybrid Crosses

  21. Mendel took true-breeding pea plants that had round yellow seeds (RRYY) and crossed them with true-breeding pea plants that had wrinkled green seeds (rryy). He already knew the round-seeded (R) trait was dominant to the wrinkled-seeded (r) trait. He also knew that yellow (Y) was dominant to green (y). All of the F1 offspring were round and yellow with the genotype RrYy. Mendel then let the F1 plants pollinate themselves. He found some plants that produced round yellow seeds and others that produced wrinkled green seeds. He also found some plants with round green seeds and others with wrinkled yellow seeds. He found they appeared in a definite ratio of phenotypes—9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green. Dihybrid Cross round yellow x wrinkled green P1 Wrinkled green Round yellow All round yellow F1 F2 9 3 3 1 Round yellow Round green Wrinkled yellow Wrinkled green Dihybrid crosses

  22. Dihybrid Cross (two traits) • P RRYY x rryy • F1 RrYy x RrYy RY Ry rY ry R r Y y All possible gamete combinations _______ _______ _______ ________ RY Ry RY rY Ry rY ry ry

  23. RY rY ry Ry RRYY RRYy RrYY RrYy RY Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy

  24. F2 genotype and ratio: 1 RRYY 2 RRYy 1 RRyy 2 RrYY 4 RrYy 2 Rryy 1 rrYY 2 rrYy 1 rryy RY rY ry Ry RRYY RRYy RrYY RrYy RY Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy

  25. F2 phenotype and ratio: 9: round, yellow 3: round, green 3: wrinkled, yellow 1 wrinkled, green RY rY ry Ry RRYY RRYy RrYY RrYy RY Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy

  26. The Principle of Independent Assortment • Mendel’s found that genes for different traits—for example, seed shape and seed color—are inherited independently of each other. • This conclusion is known as the Principle of Independent Assortment Gametes from RrYy parent Ry RY rY ry RRYy RRYY RrYY RrYy RY RRYy RRYy RrYy Rryy Ry Gametes from RrYy parent rrYy RrYY RrYy rrYY rY RrYy rrYy Rryy rryy ry

  27. A Summary of Mendel’s Principles • The inheritance of biological characteristics is determined by individual units known as genes. Genes are passed from parents to offspring. (Mendel called genes, “factors.”) • Dominance- if two alleles in a gene pair are different, the dominant allele will control the trait and the recessive allele will be hidden • Segregation - each adult has two copies of each gene-one from each parent. These genes are segregated from each other when gametes are formed. • Independent Assortment - genes for different traits are inherited independently of each other.

  28. Exceptions to Mendel’s Principles • Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. • Patterns of Inheritance • Incomplete Dominance • Codominance • Multiple Alleles • Polygenic Traits

  29. Some alleles are neither dominant nor recessive Heterozygous phenotype is somewhere in between the two homozygous phenotypes Phenotype appears to be blended but alleles remain separate & distinct There are no dominant or recessive alleles therefore uppercase and lowercase letters are not used Example: genotype of four o’clock plants red flowers = FrFr white flowers = FwFw pink flowers = FrFw Alleles for red and white flowers show incomplete dominance Heterozygous plants have pink flowers—a mix of red and white coloring Incomplete Dominance

  30. Incomplete DominanceFlower color of four o’clock plants Fr Fr X Fw Fw Cross a red plant with a white plant. Fr Fr Fw Fw FrFw FrFw Genotype: all FrFw FrFw FrFw Phenotype: all pink

  31. both alleles in the heterozygote express themselves fully and contribute to the phenotype Example: Feather color in Chickens Allele for black feather is codominant with white feathers A heterozygous phenotype is “erminette”- speckled black and white feathers It is not a blend, the colors appear separate Example: Blood Type in Humans A blood type = IAIA or IAi The letters stand for A B blood type = IBIB or IBi and B antigens. AB blood type = IAIBHumans have antigens O blood type = ii to A, B, both A and B,or neither A nor B. Codominance

  32. Although individuals can’t have more than 2 alleles, more than 2 alleles can exist in a population Example: Coat color in rabbits Coat color is determined by a single gene that has four alleles Four alleles display a pattern of dominance that produces four possible coat colors Example: Blood types in humans alleles are IA, IB, i Multiple Alleles

  33. Cross an individual with AB blood and a person with O blood Codominance and Multiple Allele Example (Blood Type in Humans) IAIB X ii IA IB Genotype: 2 IAi: 2 IBi i i IAi IBi Phenotype: 2 A: 2 B IBi IAi

  34. Polygenic Traits • results from an interaction of several genes • traits are controlled by two or more genes • Example: Eye Color of Fruit Flies • three genes make the reddish-brown pigment of fruit fly eyes • wide range of phenotypes can occur • Example: Eye Color of Humans • controlled by genes for pigment, tone, amount of pigment, and distribution of pigment

  35. Applying Mendel’s Principles • Mendel formed hypotheses about inheritance without knowing what genes are or where they are located in cells • In the 1900’s Thomas Hunt Morgan studied the fruit fly • (Drosophila melanogaster ) • easy to feed and maintain • new generation produced every 2 weeks • produce large numbers of offspring • have only four pair of chromosomes • Morgan found that Mendel’s principles applied to fruit flies as well as plants.

  36. Many doctors used Mendel’s principles to study human disorders Albinism is lack of the pigment melanin that gives human skin its color Individuals with the dominant allele (A) produce skin coloration Individuals homozygous for the recessive form of the allele (a) have albinism If two people with normal skin color have a child with albinism, what are the odds that a second child will also have albinism? If the first child has albinism (aa), then both parents must have at least one allele that is a. Since both parents have normal skin color, they must also have A. Applying Mendel’s Principles to Humans 25% Aa Aa Genotypes of the parents _______ x________ A a A AA Aa There is a 25% chance that the second child will also have albinism a Aa aa

  37. Characteristics of any organism is not determined by genes alone Characteristics are determined by the interaction between genes and the environment Genes may affect a sunflower’s height and color of its flowers, but they are influenced by climate, soil, and availability of water (if the sunflower doesn’t have enough water it can’t grow) Hydrangea with the same genotype for flower color express different phenotypes depending on the acidity of the soil. Genetics and the Environment

  38. 1903 - Walter Sutton - stated the chromosome theory of heredity - the material of inheritance is carried by the chromosomes - Sutton recognized “factors” were genes on the chromosomes he saw - both occur in pairs - both separate during meiosis - both sort independently - Sutton realized that there must be many different genes on a chromosome and that they are inherited together

  39. Gene Linkage • What we know: genes from different chromosomes sort independently in meiosis • But. . What about genes on the same chromosome? • At first, it seems they would be inherited together, but it is a little more complicated • Morgan in 1910 noticed that some genes were linked (which violates the principle of independent assortment) • Morgan and his associates studied 50 genes of the fruit fly and were able to place all of the genes in 4 linkage groups which assorted independently, but all the genes in one group were inherited together • Linked genes are located on the same chromosome and are inherited together • 4 linkage groups- fruit fly has 4 chromosomes • It is the chromosomes however, that assort independently, not individual genes

  40. Gene Linkage • Morgan discovered that chromosomes sort independently and not genes • Why didn’t Mendel discover this • 6 of the 7 traits he studied were on different chromosomes • The two genes on the same chromosomes were so far away from each other they assorted independently • Showing that genes on the same chromosomes are not forever linked and crossing over occurs • Crossing over can separate and exchange linked genes and produce new combinations of alleles. • This is important in genetic diversity of a species or population

  41. An example of gene linkage • When crossing long, purple sweet peas with round, red peas, the expected F2 ratio (9:3:3:1) did not show up • new ratio appeared to indicate that the traits did not sort independently

  42. Genetic recombination - the shuffling of genes into new combinations by crossing over during prophase I of meiosis • If genes were being sorted together, then crossing over was the only explanation for the appearance of red, long and purple, round peas • recombinant - an organism or a chromosome with a recombined set of genes

  43. Gene mapping - locating genes on chromosomes • Geneticists work with two traits at a time and look for the number of recombinants in the offspring • if two genes are far apart on a chromosome, there are more places between them where crossing over can occur, therefore more recombinants are formed • if two genes are close together, few recombinants occur

  44. Gene Mapping In 1911, Alfred Strutevant hypothesized that the rate at which crossing-over separates linked genes could be used to map were genes are located on a chromosome The farther apart genes are, the more likely they were to be separated by a cross-over Sturtevant created a gene map showing the relative location of each known gene on a chromosome

  45. The recombinant rate (frequency of crossing-over between genes) is used to construct genetic maps Genetic recombination-the shuffling of genes into new combination by crossing-over during prophase I of meiosis 50 B A 50 B A 10 5 A D B C 10 5 A D B C or or or or 10 5 A D B C 10 5 A D B C D 35 C D 35 C or or 35 C D 35 C D How to construct a gene map

  46. Suppose there are four genes— A, B, C, and D—on a chromosome. Geneticists determine that the frequencies of recombination among them are as follows: between A and B—50%; between A and D— 10%; between B and C—5%; between C and D—35%. The recombination frequencies can be converted to map units: A-B = 50; A-D =10; B-C = 5; C-D = 35. Chromosome Mapping 35 10 5 C B A D 50

  47. These map units are not actual distances on the chromosome, but they give relative distances between genes. Geneticists line up the genes as shown. The genes can be arranged in the sequence that reflects the recombination data. This sequence is a chromosome map. 50 B A 50 B A 10 5 A D B C 10 5 A D B C or or or or 10 5 A D B C 10 5 A D B C D 35 C D 35 C or 35 10 5 C B A D or 35 C D 35 C D 50 Chromosome Mapping

  48. Mapping of Earth’s Features Mapping of Cells, Chromosomes, and Genes Cell Earth Chromosome Country Chromosome fragment State Gene City People Nucleotide base pairs Comparative Scale of a Gene Map

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