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Section 2, pages 30-33. By Ivy Davis. The Tale of the Garden Pea: The Basis of Medelian Genetics.

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the tale of the garden pea the basis of medelian genetics
The Tale of the Garden Pea: The Basis of Medelian Genetics
  • In the nineteenth century “blending inheritance” was the dominant explanation for how traits were passed down, however, Charles Darwin, who spent years in the Galapagos observing and collecting data, later expressed his doubts about this idea since it was inconsistent with his findings.
  • Little did he know, a failed substitute teacher was piecing together this puzzle.
slide3

The scientific objective of Gregor Mendel’s pea plant study was to see how traits are transmitted over generations.

  • He chose the common garden pea, whether delibratly or by accident, this turned out to be a key decision.
  • Mendel started out with true-breeding varieties and chose 7 traits to investigate- height of the plant, position and color of the flower, shape and color of the pea pod, and the shape and color of the peas.
slide4

Another important decision Mendel made was to cross and follow individual plants that differ in only one trait over several generations.

  • Between 1854 and 1865, he documented the observations he made from about 30,000 plants and statistically analyzed all the data.
some important terms used in the study of genetics
Some Important Terms Used in the Study of Genetics
  • Allele One of the alternative forms of a gene
  • Autosome Any eukaryotic chromosome other than a sex chromosome
  • Chromosome A complex structure in the nucleus of eukaryotes or in the nucleoid region of
  • prokaryotes that carries DNA
  • Dihybrid cross A cross or mating in which the parents differ in two independent traits
  • Diploid Having two sets of chromosomes (one from the mother and one from the father) in a
  • sexually reproducing organism
  • DNA The genetic material for all organisms except the RNA virus
  • Dominant Describes an allele that determines the phenotype when the genotype is heterozygous
  • generation Parental generation
slide7

F1 generation The first generation of the parents

  • F2 generation The second generation of the parents
  • Gene A unit of heredity consisting of a specific sequence of nucleotides in DNA (or RNA in
  • some viruses)
  • Genotype The genetic makeup (set of alleles) of an organism
  • Haploid The condition of having one set of genes and one set of chromosomes in a sexually
  • reproducing organism
  • Heterozygous Having two different alleles at a gene locus for a particular trait
  • Homologous
  • chromosomes A pair of chromosomes with the same set of genes, each derived from one parent
  • Homozygous An individual having two copies of the same allele at a genetic locus
  • Locus (pl. loci) The location of a gene in the DNA molecule
  • Monohybrid
  • cross A cross or mating in which the parents differ in a single trait
  • Parent (P)
slide8

Phenotype The physical or biochemical characteristics of an organism that result from the

  • interaction between genotype and the environment
  • Recessive Describes an allele that does not produce a characteristic effect when present with a
  • dominant allele; the trait is expressed only under homozygous conditions.
  • Segregation The separation of alleles or homologous chromosomes during meiosis
  • Self fertilization
  • or pollination Fertilization between the sperm and the egg from the same flower
  • Sex chromosome The chromosomes that influences sex determination, e.g., XX and XY in humans
  • Test cross Mating of a dominant-phenotype individual (who may be either heterozygous or
  • homozygous) with a homozygous-recessive individual
  • True-breeding A genetic cross in which the same trait appears every time from homozygous parents
introduction to genetics
Introduction To Genetics
  • As described in Section I, in most sexually

reproducing species, (1)each chromosome inside the

nucleus of a cell is a large DNA molecule in which

many genes are located .

  • (2)Every gene codes for the production of one or more specific proteins .
  • (3)Each protein determines a particular trait, which is either expressed externally or is manifested internally as part of a complex network of metabolic reactions .
slide10

Each individual cell has a fixed number of chromosomes that is unique for that species. All chromosomes come in pairs.

  • One set comes from the father and the corresponding one comes from the mother.
  • Each of the pairs of chromosomes with the same complement of genes are called homologous chromosomes.
  • Homologous chromosomes have the same gene at the same location or locus. However, these genes may have the same or slightly different nucleotide sequence.
  • The pair of alleles that determines the expression of a particular characteristic or trait is called its genotype.
slide11

The physical or biochemical characteristic controlled by the genotype is called its phenotype.

  • A given phenotype may be determined by one or more genotypes because an allele can be either a dominant allele or a recessive allele.
  • If a pair of alleles are virtually the same, it means that the nucleotide sequences of the two alleles are the same, and therefore homozygous alleles. If they have different nucleotide sequences, they are heterozygous.
  • In general, a recessive allele implies that the gene that codes for the particular trait is mutated in such a way that its protein product is defective; so the characteristic controlled by that protein cannot be expressed.
slide13

In each diploid somatic cell in humans, there are 46 chromosomes.

  • Chromosome 1-22 are autosomal chromosomes, (Autosome-Any eukaryotic chromosome other than a sex chromosome). The 23rd pair are sex chromosomes, (sex chromosome-the chromosome that influence sex determination, e.g., XX and XY in humans)
  • Each chromosome has several hundred or thousand genes.
  • As a general rule agreed upon in the scientific community, a capital letter refers to a dominant allele and a lowercase letter refers to a recessive allele.
slide14

Mendel started out with only true breeding varieties. This is the P-parental generation.

  • The P’s offspring would be the F₁ generation, or first filial.
  • If the F₁ generation is allowed to self-fertilize, which is common in plants, the next generation would be F₂.
  • In a test cross, if the hybridization experiment investigates only a single trait at a time, it is called a monohybrid cross. A dihybrid cross investigates two traits simultaneously.
slide15

During the time of Mendel’s experiments, most of these terms were not used by the scientific community. The only tool available for Mendel was an experimental protocol for test crossing true breeding plants and mathematical rules for predicting probability that could be used in analyzing data.

  • Mendel was the first person to systematically apply mathematical principles in predicting and analyzing results in the study of genetics. This ultimately led him to develop the laws of genetics.