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Chapter 2: Genetics

Chapter Outline

     

    1. Mendel’s Genetics Experiments

    1. Pea plants
      • From 1856 to 1863, Mendel conducted a series of experiments with the common pea.
      • He chose two variants to study: color and texture.
    2. Experiment results
      • An organism’s phenotype (observed characteristics) is determined jointly by particles inherited from the mother and the father.
      • Each particle is equally likely to be passed on to the next generation (independent assortment).
      • Mendel published his results in 1866, but they were dismissed by leading botanists of the time.
      • By the end of the nineteenth century, other botanists were repeating Mendel’s experiments with other plant species.

    2. Cells and Chromosomes

    1. Mitosis and meiosis
      • The division of cells and their nuclei is called mitosis.
      • During mitosis, chromosomes can be seen.
      • Diploid organisms, like primates, have chromosomes in homologous pairs.
      • Meiosis is the cell division that produces gametes, or the eggs and sperm.
      • Haploid cells, like sex cells, contain only one copy of each chromosome.
      • Reproduction joins a haploid sperm and a haploid egg to create a diploid zygote.
    2. Genes
      • Genes are particles carried on chromosomes. Collectively, all of the genes on a chromosome are known as the genome.

      • Alleles are variants of genes.
      • Individuals with two of the same allele are homozygous, while individuals with two different alleles are heterozygous.
      • The set of genes that an individual carries is the genotype, while the outward expression of those genes is the phenotype.
      • When only one copy of an allele is necessary to create a change in an organism’s phenotype, the allele is called dominant.  If two copies are needed, it is said to be recessive.
      • A Punnet square can be constructed to predict the genotypes of offspring (Figure 2.10).
    3. Linkage and recombination
      • New traits are produced during “shuffling” of the genetic code, a process called recombination.
      • Crossing over creates chromosomes in an offspring that have gene combinations not present in the parent.

    3. Molecular Genetics

    1. How DNA works
      • Watson and Crick discovered the structure of DNA in 1953.
      • DNA is composed of sequences of phosphate and sugar molecules with one of four bases attached: adenine, guanine, cytosine, thymine.
      • Copying of DNA is accomplished by unzipping the double-helix strands, adding complementary bases, and proofreading the result.
    2. DNA encoding
      • Protein-coding genes specify the structure of proteins such as enzymes, which regulate the biomechanics of an organism.
      • Regulatory genes determine when the message in the DNA will be expressed.
      • Some DNA codes for RNA molecules.
    3. Protein encoding
      • Examples of proteins include keratin (hair) and collagen (bone).
      • Proteins are composed of strings of amino acids.
      • The protein’s function depends on which amino acids make it up.
      • DNA is combined into three-letter sequences (codons) that code for amino acids.
      • Redundancy is built into this system to prevent random change.
      • DNA has to be translated into RNA before protein synthesis.
      • Ribosomes read the messenger RNA (mRNA) copy and create a protein.
    4. Coding and noncoding DNA
      • Organisms that lack chromosomes and nuclei are called prokaryotes.
      • Those with chromosomes and nuclei are eukaryotes.
      • Study of eukaryotes revealed that noncoding sequences called introns interrupt DNA.
      • Only about 2% of the genome is composed of exons, or coding sequences.

     

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