Lecture 4: Beyond Mendelian Genetics

• Mendelian Genetics


  Theory proposed by Mendel that explains the rules governing heredity in terms of chromosomes and genes
• Mendel's theory

  • Exquisite predictive power
  • Explains discrete traits, not continuous traits
  • Requires all genes to be on different chromosomes, but there are only 23 pairs of human homologs
• Sex linked traits


  Traits that run in families but predominantly affect males
• Inheritance of color blindness

  1. Recessive allele on Y chromosome

  2. Colorblind man x normal woman: all children normal, females heterozygous

  3. Colorblind woman x normal man: all male children colorblind, all female children heterozygous
• Hemizygous


  Situation where a gene is missing from one of the chromosomes
• Wild type


  Functional allele
• Mutant allele


  Defective allele
• Polymorphic gene


  Gene with multiple common alleles in a population
• Crossing over during prophase of meiosis1


  1. Chromatids exchange arms

  2. Can result in recombination of linked genes
• Recombination frequency


  Measure of distance between two genes on the same chromosome
• Linkage group


  Set of genes on the same chromosome
• Incomplete dominance


  Heterozygotes have an intermediate phenotype between the homozygotes, example of snapdragon and flowers red, white or pink
• Expressivity


  Degree to which individuals of the same genotype vary in the expression of a phenotype
• Penetrance


  Percentage of individuals with a given genotype that express the expected phenotype
• Polygenic/Multigenic traits


  Traits influenced by multiple genes
• if genes are linked: alleles can be in cis (alleles on the same homologous chromosomes) or trans (alleles on different homologous chromosomes)