Biology 301

 Variance

1. Importance to concept of natural selection

2. Differentiation of variance components which are genetically determined versus components affected by the environment of the organism

3. Terminology:
     Genes - physical entity transmitted from parent to offspring that influences hereditary traits.
     Loci - specific locations on DNA where genes reside
     Alleles - alternative forms at one locus
     DNA
          Codons - 3 base pair units corresponding to tRNA - each codes for an amino acid, or to start or stop codes for
          'reading frames.'
          Introns - non-coding regions
          Exons - coding regions
     Chromosomes - DNA containing bodies in cells
     Diploid organisms have 2 copies of most genes

  •           homozygous - same allele on both copies
  •           "homo" = like; "zygous" = offspring
  •           heterozygous - different allele on each copy
  •           "hetero" = different; "zygous" = offspring

  •      Genotype - Allele distribution in an organism
  •           Symbolism: letter symbolizes locus, capital or lower case symbolizes allele
  •                A = allele 1 at locus 1, a = allele 2 at locus 1
  •                B = allele 1 at locus 2, b = allele 2 at locus 2

  •           example of 2 locus system:
     
    AA BB AA Bb AA bb
    Aa BB Aa Bb Aa bb
    aa BB aa Bb aa bb

     
         Phenotype - physical expression of genotype including dominance, recessiveness, additivity, etc.

              Example: Additive action: Pigmentation = phenotype
     
    Genotype Pigmentation
    AA BB 4
    Aa BB  or  AA Bb 3
    aa BB  or  Aa Bb  or  AA bb 2
    Aa  bb  or  aa bB 1
    aa bb 0

       4. Examples of looking for variation in phenotype

    a. external morphology
              Allele Frequency Calculations
     
    Genotype FF FS SS Total
    No. of individuals 8 6 2 16
    No. of F fast alleles 16 6 0 22
    No. of S (slow) alleles 0 6 4 10
    Total alleles 16 12 4 32
              Genotype frequency calculations           Polymorphism --> most common allele frequency < 0.95

              The greater the fraction of polymorphic genes in a population, the more genes that are expected to be heterozygous.
     
              c. Measuring variation at the DNA level --> direct measurement of genetic variation

                        Restriction enzymes cut DNA at sites of specific nucleotide sequences

                        EcoR1 cuts at AATT
                        BamH1 cuts at GATC
                        Xho1 cuts at TCGA
                        Cut DNA with restriction enzyme, electrophorese it, transfer to nitrocellulose filter.
                        Hybridize with radioactive DNA from the region of interest in the genome.
                        Expose photographic emulsion to filter, develop with film developer. Dark bands represent the
                        polymorphism
                        Read the bands
                             Long DNA --> uncut, 1 band, migrates slowly
                             Short DNA --> cut, 2 bands, migrate faster

                   Difficult and expensive to run large numbers of samples from many individuals.
     

    Natural Selection--mechanism by which evolution occurs

    1. Operates on the individual phenotype, NOT the genotype
    2. Must be variation in the phenotype which is inherited to see a change in phenotype in the next generation
    3. Occurs when individuals differ in fitness
    4. Fitness is determined by a genotype's contribution to the next generation and thus its survival and reproduction
    5. Typically represent the distribution of phenotypes in a population as a frequency diagram

    Types of Natural Selection

    1. Stabilizing--selective advantage to the intermediate form
    2. Disruptive--selective advantage to extremes
    3. Directional--selective advantage to one of the extremes

    Outcome of Selection

    1. Depends on the relationship between the phenotype and the genotype
    2. Discuss outcomes in terms of relative fitness = the average contribution of one genotype to the next generation compared to that of other genotypes
    3. Contribution is determined by survival and reproduction
    4. w = fitness
    5. Typically designate w11 as the relative fitness of AA, w12 as the relative fitness of Aa, and w22 as the relative fitness of aa
    6. If assume a direct correspondence between the phenotype and the genotype, then after selection the ratio of genotypes will be p2w11 : 2pqw12 : q2w22
    7. Similarly, after selection the ratio of alleles will be   p(pw11 + qw12) : q(pw12 + qw22)
    8. Using these relationships one can show that changes in allele frequencies result from different fitnesses of the genotypes
    9. Time necessary for changes in allele frequencies depends on the selection coefficient (s) and the degree of dominance (h)
    10. Recessive alleles change slowly in frequency when they are rare while dominant alleles change slowly in frequency when they are common
    11. Can have heterozygote superiority so that w12 > w11 or w22 (human sickle cell trait in Africa) or heterozygote inferiority where w12 < w11 or w22 (unstable so typically get fixation of one or the other allele)