Mendelian Genetics

• What are Mendel's three laws of inheritance?

  Law of Segregation, Law of Independent Assortment & Law of Dominance.
• What is the Law of Segregation?

  • Every individual organism contains two alleles for each trait.
  • These alleles segregate (seperate) during meiosis.
  • Each gamete contains only one of the alleles.
• What is an allele?

  A different version of a gene.
• What is a Genotype?

  Alleles of a particular gene or genes, present in a particular individual.
• What is a phenotype?

  Expression of the genotype

  • Morphology
  • Biochemistry
  • Behaviour
• Are there exceptions to Mendel's first law?

  • Prokaryotes (bacteria and archaea).
  • Clonal reproduction
  • Haploid organism
  • Sex chromosomes.
• What is the Law of Independent Assortment?

  Alleles for seperate traits are passed independently of one another from parents to offspring.
• Are there exceptions to Law of Independent Assortment?

  • Clonal Reproduction - entire genome is inherited.
  • Genes on the same chromosome - more likely to be inherited together (linkage).
• What is the Law of Dominance?

  • Recessive alleles will always be masked by dominant alleles.
  • Complete dominance - phenotype of recessive allele is only expressed when homozygous.
• Are there exceptions to the Law of Dominance?

  • Incomplete dominance - phenotype of heterozygote is intermediate between two homozygotes. e.g. red, pink & white flowers.
  • Codominance - phenotype of heterozygote contains traits of both alleles e.g. white, black & speckled chickens.
• Probability


  Probability of event 1 happening is x

  Probability of event 2 happening is y

  Probability of both events happening is xy

  Probability of either event happening is x+y
• What is Epistasis?

  • 2+ genes interact to affect a single trait.
  • E.g. Epistatic white gene in cats (dominant epistasis), dominant allele W^D causes white colour (and blue eyes and deafness) regardless of colour alleles at other genes.
• Agouti cat colours

  • 'A' tabby
  • 'a' solid coat (markings hidden)
• Recessive epistasis


  explaining the result with mice.

  
• What are human examples of Mendelian and Quantitative traits?

  • Mendelian - Dry vs sticky ear wax
  • Quantitative - height, skin colour.
• How does no. of genes contributing to a trait affect genotypes?

  As number of genes contributing to a trait increases• Number of genotypes increases• Distribution of trait values approaches normal distribution