BIOL 151 Final: Chapter 1 Notes
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Department
Biology
Course
BIOL 151
Professor
Chris Felege
Semester
Spring

Description
Mendel and the Gene Gregor Mendel • A19th century Monk • Active member of his city's agricultural society • Interested in heredity • In 1885, Mendel worked out the rules of inheritance through a series of experiments on garden peas • Early in the 20th century, Walter Sulton and Theodor Boveri formulated the chromosome theory of inheritance ◦ Proposes that meiosis causes the patterns of inheritance that Mendel observed. • Genetics is the branch of biology that focuses on inheritance • Heredity is the transmission of traits from parents to their offspring • Traits are any characteristic of an individual • Mendel was addressing the basic questions ◦ Why offspring resemble their parents ◦ How transmission of traits occurs • In his time, two hypotheses had been formulated to try to answer the questions: 1. Blending Inheritance i. Parental traits blend ii. Such that their offspring have intermediate traits 2. Inheritance of acquired characteristics i. Parental traits are modified ii. Then passed to their offspring • Genetics uses model organisms because the conclusions drawn from them can be applied to other species • Mendel chose the common garden pea (Pisum sativum) as his model organism because: ◦ It is easy to grow ◦ Its reproductive cycle is short ◦ It produces a large number of seeds ◦ Its mating is easy to control ◦ Its traits are easily recognizable • Mendel did not know however, that all the traits that he studied ended up being very far apart on different chromosomes or on different chromosomes all together, so he didn't have to worry about things like linkage • Peas normally pollinate themselves ◦ Self-fertilization • Mendel could prevent self pollination by removing the male reproductive organs containing pollen from each flower • He then used this polled to fertilize the female reproductive organs of flowers on different plants, thus performing cross-pollination • Mendel worked with pea varieties that differed in seven easily recognizable traits ◦ Seed shape ◦ Seed colour ◦ Pod shape ◦ Flower colour ◦ Flower and pod position ◦ Stem length ◦ Pod colour • An individuals observable features comprise its phenotype • Mendel worked with pure lines ◦ Produces identical offspring when self-pollinated • He used these plants to create hybrids ◦ He mated two different pure lines that differed in one or more traits • The adults in a cross are the parental generation • The offspring are the F1 generation ◦ The F indicates "First filial" • When Mendel allowed the F1 generation to self-pollinate, an F2 generation was created ◦ F2 generation is the "Second filial" generation • The terms dominant and recessive identify only which phenotype is observed in individuals carrying two different genetic determinants Definitions • Gene ◦ Ahereditary factor that influences a particular trait •elllA ◦ Aparticular form of a gene • Genotype ◦ Alisting of the alleles in an individual • Phenotype ◦ An individuals observable traits • Homozygous ◦ Having two of the same allele • Heterozygous ◦ Having two different alleles • DominantAllele ◦ An allele that produces its phenotype in heterozygous and homozygous form • RecessiveAllele ◦ An allele that produces its phenotype only in homozygous form • Pure Line ◦ Individuals of the same phenotype that, when crossed, always produce offspring with the same phenotype • Hybrid ◦ Offspring from crosses between homozygous parents with different phenotypes • Reciprocal Cross ◦ Across in which the phenotypes of the male and female are reversed compared with a prior cross • Testcross ◦ Across between a homozygous recessive individual and an individual with the dominant phenotype but unknown genotype • X-linked ◦ Referring to a gene located on the X chromosome • Y-linked ◦ Referring to a gene located on the Y chromosome • Autosomal ◦ Referring to a gene located on any non-sex chromosome (an autosome) or a trait determined by an autosomal gene • Mendel wanted to determine if gender influenced inheritance ◦ He performed a reciprocal cross ◦ He established that it does not matter whether the genetic determinants are located in the male or female parent • Mendel proposed a hypothesis called particular inheritance ◦ It suggests that hereditary determinants maintain their integrity ◦ From generation to generation ◦ This directly contradicts the idea of blending inheritance and the inheritance of acquired characteristics hypotheses • Hereditary determinants for a trait are now called genes • Mendel also proposed that: ◦ Each individual has two versions of each gene ◦ These different versions of a gene are called alleles • Different alleles are responsible for the variation in the traits that Mendel studied • The allele found in an individual are called its genotype • An individual's genotype has a profound effect of its phenotype • 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 ◦ Uppercase letter was dominant ◦ Lowercase letter was recessive • Individuals have two alleles of each gene ◦ Two copies of the same allele • Are homozygous ▪ RR or rr ◦ Two different alleles • Are heterozygous ▪ Rr The Monohybrid Cross • Amating of two heterozygous parents ◦ Results in offspring that are 1/4 RR, 1/2 Rr, and 1/4 rr ◦ Produces a 3:1 ratio of phenotypes • 3:1 phenotypic ratio • 1:2:1 genotypic ratio • Mendel's genetic model ◦ Aset of hypotheses that explain how a particular trait is inherited ◦ Explains the results of these crosses • A Punnett Square ◦ Is now used to predict the genotypes and phenotypes of the offspring from a cross • Mendel used Dihybrid Crosses ◦ Matings between parents that are both heterozygous for two traits ◦ Used to determine whether the principle of segregation hold true if parents differ in more than one trait • Mendel's experiments tested two contrasting hypotheses: 1. IndependentAssortment i. Alleles of different genes are transmitted independently of each other 2. DependentAssortment i. The transmission of one allele depends on the transmission of another ◦ Mendel's results supported the principle of independent assortment • The Punnett Square from a dihybrid cross predicts: ◦ There should be 9 different offspring genotypes and 4 different phenotypes ◦ The phenotypic ratio will always be 9:3:3:1 with two heterozygous parents • Mendel accepted the hypothesis that: ◦ Alleles of different genes are transmit
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