Outline and Chapter notes to accompany chapter 3

                            HUMAN GENETICS
                                                               Dec., 2003

A.  WHAT DO GENES DO?

  GENE EXPRESSION consists of two steps:

    TRANSCRIPTION to RNA:  DNA unwinds in one area, then part of one
       DNA strand can be used as a template to synthesize a complementary
       strand of RNA, still keeping the message in the language
       of a series of nucleotides.

    TRANSLATION into protein (i.e., changing nucleotide language
          into a sequence of amino acids, the language of proteins):
       Within a ribosome, MESSENGER RNA comes together with a
       TRANSFER RNA molecule linked to an amino acid.  A three-base
       sequence (CODON) on the messenger RNA matches a complementary
       sequence (ANTICODON) on the transfer RNA.  The codon thus determines
       which transfer RNA molecule is used, and thus which amino acid is
       the next one to be inserted in a growing polypeptide strand.

  MUTATIONS:
    Mutations are heritable changes in genes or chromosomes.
    Most mutations are SINGLE-GENE MUTATIONS that arise from errors in
        replication or from unrepaired damage to DNA molecules.   
        NOTE:  Some geneticists restrict the term "mutation" to
          single-gene mutations only.
    Single-gene mutations include:
        base-pair substitutions
        frame-shift mutations (additions & deletions)
    CHROMOSOMAL ABERRATIONS, which arise from errors in meiosis, include:
        changes in chromosome number (including polyploidy in plants)
        duplications:  a piece of a chromosome appears twice
        inversions:  a piece of a chromosome turns 180 degrees
        deletions:  a piece of a chromosome is missing
        translocations:  a piece of a chromosome attaches to another
            chromosome


B.  SOME DISEASES AND DISEASE PREDISPOSITIONS ARE INHERITED

  IDENTIFYING GENETIC CAUSES FOR TRAITS.
    PEDIGREES of large families (or of many families) can help identifiy
      whether a genetic trait is caused by a dominant or a recessive
      allele, and whether the trait is sex-linked or autosomal.
    Many human traits are affected by single genes.  Examples include:
      Brown eyes (dominant to blue eyes)
      Ability to curl or roll up tongue (dominant to inability)
      Ability to taste the chemical PTC (dominant to nontasting)

  SOME HEREDITARY DISEASES ARE ASSOCIATED WITH KNOWN GENES:
      Huntington's disease (dominant to absence of the disease)
         NOTE: phenotypic effects develop late in life
      Metabolic diseases controlled by recessive alleles are called
         "INBORN ERRORS OF METABOLISM"
        Albinism (inability to make melanin pigment) 
        Phenylketonuria (PKU) (inability to break down the amino acid
            phenylalanine)
        Alkaptonuria (inability to break down homogentisic acid)
        Porphyria (inability to break down certain purines)


C.  GENETIC INFORMATION CAN BE USED OR MISUSED IN VARIOUS WAYS.

  GENETIC TESTING AND COUNSELING:
    Pedigree analysis helps identify dominant and recessive traits; it
        also distinguishes sex-linked traits from other (autosomal) traits.
    Identifying a gene (or its gene product):  DNA probes for specific
        genes;  enzyme tests for many gene products.  Some of these
        tests are preceded by PCR (polymerase chain reaction) to get
        enough identical copies of DNA to run the tests.
    Identifying chromosome abnormalities
    Newer sampling methods include:  AMNIOCENTESIS, CHORIONIC VILLUS
        SAMPLING
    Some tests allow us to detect heterozygous carriers of recessive
        traits;  others do not.
    We can advise testees of future risks for themselves or their
        children.
    Most testing requires obtaining INFORMED CONSENT first
    Genetic testing raises many new ethical issues in medical decision-
        making

  ALTERING INDIVIDUAL GENOTYPES:
    Currently possible only through genetic engineering (RECOMBINANT
        DNA THERAPY)

  ALTERING THE GENE POOL OF POPULATIONS:
    POSITIVE EUGENICS = encouraging certain genotypes to breed in
        greater numbers
    NEGATIVE EUGENICS = preventing certain genotypes from breeding (by
        sterilizing or killing them)--  has led to genocide in the past
    CLONING (not currently possible with humans) = asexually produced
        (therefore genetically identical) organisms or cells
    An ethical question:  who decides what is considered a defect? 

  CHANGING THE BALANCE BETWEEN GENETIC AND ENVIRONMENTAL FACTORS:
    Euphenics:  modification of individual phenotypes (producing
        PHENOCOPIES)
    Euthenics:  providing external assistive devices (like wheelchairs
        & eyeglasses)
    Eupsychics:  changing other peoples' social attitudes, customs, and
        laws, or providing special education for handicapped
        individuals

                  ----------------------------------           Dec., 2003



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