Mendelian Genetics

• Incomplete Dominance

  When the heterozygous condition expresses a third phenotype different from either homozygous condition

  Ex. red flower and white flower making pink flower.
• Codominance

  When the heterozygous condition expresses both of the homozygous conditions simultaneously

  Ex. Cows have one red hair and one white hair not pink hair.
• Codominance Example

  The body makes anti-blood type antibodies for any carbohydrate that is not on the surface of its own blood cells

  These antibodies cause blood cells that have particular carbohydrates to agglutinate (clump)

  Transfusion with the wrong blood type will kill a person
• Multiple Alleles

  When there are more than two alleles for a gene and multiple alleles control a trait

  Ex. Human Blood Types

  3 blood type alleles = 6 possible genotypes, 4 different phenotypes
• Polygenic

  when more than one gene contributes to the expression of a particular trait

  Ex. Human Skin Colour
• Complex Interactions

  The interaction among genes and the organism can be highly variable

  Multiple genes controlling multiple locations
• Pleiotropy

  When one gene has multiple affects on the physiology of an organism

  Ex. Sickle Cell Anemia
• Epistasis

  When two or more genes control expression of a single trait

  Ex. Coat Colour in Labrador Retrievers
• Penetrance

  The likelihood of a genotype expressing a phenotype an organism
• Expressivity

  The extent to which a trait is expressed in an organism
• Sex Linkage

  • Refers to genes that are on sex chromosome (x,y)
  • Males will show sex linked traits at a higher frequency than females because they have only one X and one y chromosome and they express all the genes from both of them
  • females express one X ( codominance, blending)
• Gene Linkage

  • Refers to any genes that are on the same chromosome
  • Usually linked genes will travel together during meiosis
• Linkage Mapping

  • Since linked genes are only separated by crossing over events the higher the frequency of recombinant offspring for any 2 genes the greater the likelihood of there being a crossing over event between them
  • This correlates to a greater distance between the jeans on a chromosome
  • This information can be used to generate a linkage map of genes on a chromosome
  • Map unit indicate distance between genes on a linkage map
  • One map unit = 1% chance of crossover and recombinant offspring
• Extra Nuclear Genes

  • Mitochondria and chloroplasts have their own chromosomes
  • These are inherited matrilineally
  • In males the Y chromosome is inherited patrilineally
• Epigenetic

  Refers to any heritable trait that is not determined by DNA sequences but determined by the environment
• Genomic Imprinting

  Some traits are only expressed if the genes are inherited from one parent or the other
• Methylation

  • Methyl groups are added to DNA to activate it
  • Evidence suggest this methylation pattern maybe uniquely inheritable and affect phenotype only immediately in successive generations
• Environmental Influence

  The interaction of an organisms genome and it's environment leads to complex patterns of gene expression