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

 
     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

• Spot patterns, color patterns.

b. allozymes by electrophoresis (Allozymes are enzymes that differ in electrophoretic mobility as a result of allelic differences at a single locus. )
 
•                Monomeric enzymes --> fast vs slow mobility in electric field. One or 2 bands for each individual.
•                Dimeric enzymes --> 3 kinds of dimers: fast/fast, fast/slow, slow/slow. 3 bands for each individual.

•           Allele frequency of F = 22 / 32 = 0.6875
•           Allele frequency of S = 10 / 34 = 0.3125

•           Genotype frequency of FF = 8/16 = 0.5
•           Genotype frequency of FS = 6/16 = 0.375
•           Genotype frequency of SS = 2/16 = 0.125

          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 Fragment Length Polymorphisms (RFLPs)

                    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

•                DNA sequencing --> much more laborious than RFLP analysis, but gives better measure of genetic variation.

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)