Mendel and the Gene PDF

Mendel and the Gene Early Genetics Genetics is the branch of biology that focuses on inheritance. This discipline deals with the manner in which genetic differences among individuals are passed from generation to generation. Heredity is the transmission of traits from parents to their offspring. A t...

Mendel and the Gene Early Genetics Genetics is the branch of biology that focuses on inheritance. This discipline deals with the manner in which genetic differences among individuals are passed from generation to generation. Heredity is the transmission of traits from parents to their offspring. A trait is any characteristic of an individual. Genes What genetic principles account for the passing of traits from parents to offspring? Lecture Outline 1. Mendel’s Experiments a) b) c) d) e) Models of Inheritance Peas as a Model Organism Monohybrid Cross and the Principle of Segregation Dihybrid Cross and the Principle of Independent Assortment Test Cross 2. Probability Rules a) Multiplication Rule b) Addition Rule Lecture Outline 1. Mendel’s Experiments a) b) c) d) e) Models of Inheritance Peas as a Model Organism Monohybrid Cross and the Principle of Segregation Dihybrid Cross and the Principle of Independent Assortment Test Cross 2. Probability Rules a) Multiplication Rule b) Addition Rule Inheritance Hypotheses The blending hypothesis is the idea that parental traits blend so that their offspring have intermediate traits. e.g., blue and yellow paint blend to make green. The inheritance of acquired characteristics hypothesis is that parental traits are modified through use and then passed on. The particulate hypothesis is the idea that parents pass on discrete heritable units (genes). ‒ Mendel documented a particulate mechanism and discovered the basic principles of heredity by breeding garden peas in carefully planned experiments. Lecture Outline 1. Mendel’s Experiments a) b) c) d) e) Models of Inheritance Peas as a Model Organism Monohybrid Cross and the Principle of Segregation Dihybrid Cross and the Principle of Independent Assortment Test Cross 2. Probability Rules a) Multiplication Rule b) Addition Rule Mendel’s Model Organism Mendel chose peas because they: Were inexpensive and easy to grow. Have a short generation time. Produce large numbers of seeds. Can control which parents were involved in a mating. Have several polymorphic traits. Trait that appears commonly in two or more different forms (e.g., purple versus white flowers). Pea Plant Mating Peas normally selffertilize (self-pollinate). Mendel could prevent self-pollination by removing the male organs from a flower. He used pollen from one flower to fertilize another. This is called a cross, or cross-pollination. Polymorphic Traits of Pea Plants Mendel worked with pea varieties that differed in seven easily recognizable traits: Seed shape, seed colour, pod shape, pod colour, flower colour, flower and pod position, and stem length An individual’s observable features comprise its phenotype. Mendel’s pea population had two distinct phenotypes for each of the seven traits. Mendel’s Experiments Mendel worked with pure lines. o They produced offspring identical to themselves when self-fertilized. o Called the parental (P) generation. He used these plants to create hybrids. o He mated two different pure lines that differed in one or more traits. o The hybrids were called the F1 generation. F1 plants were allowed to self-pollinate to create the F2 generation. ? ? Lecture Outline 1. Mendel’s Experiments a) b) c) d) e) Models of Inheritance Peas as a Model Organism Monohybrid Cross and the Principle of Segregation Dihybrid Cross and the Principle of Independent Assortment Test Cross 2. Probability Rules a) Multiplication Rule b) Addition Rule The Monohybrid Cross Mendel’s first experiments involved crossing pure lines that differed in just one trait. Dominant and Recessive Traits Mendel called: The genetic determinant for wrinkled seeds recessive. The determinant for round seeds dominant. Mendel repeated these experiments with each of the other traits. In each case, the dominant trait was present in a 3:1 ratio over the recessive trait in the F2 generation. Particulate Inheritance Mendel’s results were inconsistent with both the blended inheritance and the acquired characters hypotheses. He proposed a hypothesis of particulate inheritance. Hereditary determinants do not blend or change through use. They act as discrete, unchanging particles. Genes, Alleles, and Genotypes Hereditary determinants for a trait are called genes. Mendel also proposed that: Each individual has two copies of each gene. Different versions of a gene are called alleles. Different alleles are responsible for variation in traits. The combination of alleles found in an individual is called its genotype. An individual’s genotype has a profound effect on its phenotype. Principle of Segregation Mendel developed the principle of segregation. The two members of each gene pair must segregate. They separate into different gamete cells during the formation of eggs and sperm in the parents. Mendel used a letter to indicate the gene for a particular trait. E.g., dominant trait is R and recessive is r. Individuals have two alleles of each gene. Individuals with two copies of the same allele are homozygous. RR or rr Individuals with two different alleles are heterozygous. Rr Monohybrid Cross A monohybrid cross is a mating between two parents that are both heterozygous for one gene. Review Question A line of pea plants, Pisum hypotheticalis, with small flowers was crossed with another line that had large flowers. The offspring of this cross (the F1) all had small flowers. Give the genotypes for each of the parents and the F1 generation. Give the resulting genotypic and phenotypic ratios for the F2 generation. In a simple Mendelian monohybrid cross, true-breeding tall plants are crossed with short plants, and the F1 plants, which are all tall, are allowed to self-pollinate. What fraction of the F2 generation are both tall and heterozygous? A. B. C. D. E. 1/8 1/4 1/3 2/3 1/2 In a simple Mendelian monohybrid cross, true-breeding tall plants are crossed with short plants, and the F1 plants, which are all tall, are allowed to self-pollinate. What fraction of the F2 generation are both tall and heterozygous? A. B. C. D. E. 1/8 1/4 1/3 2/3 1/2 Lecture Outline 1. Mendel’s Experiments a) b) c) d) e) Models of Inheritance Peas as a Model Organism Monohybrid Cross and the Principle of Segregation Dihybrid Cross and the Principle of Independent Assortment Test Cross 2. Probability Rules a) Multiplication Rule b) Addition Rule The Dihybrid Cross Mendel used dihybrid crosses to determine whether alleles of different genes segregate together or independently. A dihybrid cross is a mating between parents that are both heterozygous for two traits. Tested two contrasting hypotheses: 1. Independent assortment: alleles of different genes are transmitted independently of each other. 2. Dependent assortment: the transmission of one allele depends on the transmission of another. Dihybrid Cross Mendel’s results supported the principle of independent assortment. Therefore, alleles of different genes are transmitted independently of one another. Review Question In another experiment with Pisum hypotheticalis, a line with small purple flowers was crossed with a line with large white flowers. All offspring of this cross (the F1) had small purple flowers. What are the genotypes of the P and F1 generation? What would be the expected genotypic ratio and phenotypic frequencies from an F1 x F1 cross (assuming random assortment of alleles during meiosis)? Pea plants heterozygous for flower position and stem length (AaTt) are allowed to self-pollinate, and 400 of the resulting seeds are planted. How many offspring would be predicted to have terminal flowers (aa) and be dwarf (tt)? AT At aT at AT AATT AATt AaTT AaTt At AATt AAtt AaTt Aatt aT AaTT AaTt aaTT aaTt at AaTt Aatt aaTt aatt Phenotypic ratio: 9 axial flowers; tall 3 axial flowers; dwarf 3 terminal flowers; tall 1 terminal flowers; dwarf 25 plants would be predicted to have terminal flowers and be dwarf. Lecture Outline 1. Mendel’s Experiments a) b) c) d) e) Models of Inheritance Peas as a Model Organism Monohybrid Cross and the Principle of Segregation Dihybrid Cross and the Principle of Independent Assortment Test Cross 2. Probability Rules a) Multiplication Rule b) Addition Rule Testcross How can we determine the genotype of an individual with the dominant phenotype? In a testcross a homozygous recessive parent is mated with a parent that has the dominant phenotype but an unknown genotype. The genotype of the unknown parent can be inferred from the results. Which statement about a testcross is not true? A. It tests whether an unknown individual is homozygous or heterozygous. B. The test individual is crossed with a homozygous recessive individual. C. If the test individual is heterozygous, the progeny will have a 1:1 ratio. D. If the test individual is homozygous, the progeny will have a 3:1 ratio. E. Testcross results are consistent with Mendel’s model of inheritance for unlinked traits. Which statement about a testcross is not true? A. It tests whether an unknown individual is homozygous or heterozygous. B. The test individual is crossed with a homozygous recessive individual. C. If the test individual is heterozygous, the progeny will have a 1:1 ratio. D. If the test individual is homozygous, the progeny will have a 3:1 ratio. E. Testcross results are consistent with Mendel’s model of inheritance for unlinked traits. Which statement about an individual that is homozygous for an allele is not true? A. Each of its cells possesses two copies of that allele. B. Each of its gametes only contains one copy of that allele. C. It is true-breeding with respect to that allele. D. Its parents were necessarily homozygous for that allele. E. It can pass that allele to its offspring. Which statement about an individual that is homozygous for an allele is not true? A. Each of its cells possesses two copies of that allele. B. Each of its gametes only contains one copy of that allele. C. It is true-breeding with respect to that allele. D. Its parents were necessarily homozygous for that allele. E. It can pass that allele to its offspring. Lecture Outline 1. Mendel’s Experiments a) b) c) d) e) Models of Inheritance Peas as a Model Organism Monohybrid Cross and the Principle of Segregation Dihybrid Cross and the Principle of Independent Assortment Test Cross 2. Probability Rules a) Multiplication Rule b) Addition Rule Probability and Mendelian Inheritance Mendel’s principles of segregation and independent assortment reflect the rules of probability. Probability and Mendelian Inheritance The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities. Probability and Mendelian Inheritance The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities. Probability and Mendelian Inheritance We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple traits. Review Question In the genetic cross, AaBbCcDdEE x AaBBCcDdEe, what proportion of the offspring will be heterozygous for all of the genes (AaBbCcDdEe)? Assume all genes are unlinked and the alleles show simple dominance. Probability and Mendelian Inheritance In the cross PpYyRr x Ppyyrr, what fraction of offspring would be predicted to exhibit the recessive phenotypes for at least two of the three characters? Review Question Imagine that you are a genetic counselor, and a couple planning to start a family comes to you for information. Charles was married once before, and he and his first wife had a child with cystic fibrosis. The brother of his current wife, Elaine, died of cystic fibrosis. What is the probability that Charles and Elaine will have a baby with cystic fibrosis? (Neither Charles, Elaine, nor their parents have cystic fibrosis.)