Mendelian inheritance

• Genetics: Mendel 1822-1884 'The Father of Genetics', 'Genetics' coined by Bateson in 1905, 'Gene' coined by Johannsen in 1909
• Genetics and Society

  • Cloning
  • Stem cell research
  • Genetic testing
  • Gene therapy
  • DNA fingerprinting
  • Health
  • Medicine
  • Family Life
  • Ethics
  • Insurance
  • Law
• Gregor Mendel (1822-1884)


  The father of genetics
• Mendel's work with peas led to

  • The discovery of dominant and recessive traits
  • The concept of the gene ('heritable factor')
  • The formulation of the basic laws of inheritance
• Character

  A heritable feature of an individual
• Trait


  A variant form of a character (the phenotype)
• Mendel's first law: The Law of Segregation

  "The two forms of a gene (alleles) present in each parent segregate independently"
• Monohybrid cross


  A cross between two true-breeding individuals differing in only one character
• Mendel's observations in monohybrid crosses: All the F1 progeny resembled one of the parents, but both of the original parental traits appeared in the F2 generation in a ratio of ~3:1
• Recessive

  The 'heritable factor' for the recessive trait had not been lost in the F1 – just masked by the presence of the factor for the dominant trait
• Alleles


  Variations in inherited characteristics are due to the existence of alternative versions of heritable factors ('genes')
• For each character, an organism inherits two alleles, one from each parent
• Phenotype


  If the two alleles differ, then the dominant allele determines the organism's appearance
• The alleles do not blend when present in the same individual – they remain discrete
• Segregate

  The two alleles segregate (separate) during gamete formation - and end up in different gametes
• Mendel's Law of Segregation

  "The two forms of a gene (alleles) present in each parent segregate independently"
• Each parent has two alleles - but only one is passed to an individual offspring via the gamete (pollen or egg)
• Each offspring receives one allele from one parent and the other allele from the other parent
• Punnett square


  A diagrammatic device for predicting the outcomes of crosses between parents of known genotype
• There are three possible combinations of two alleles
• Genotype


  Different genotypes can produce the same phenotype
• Testcross


  A method for determining the genotype of an individual with the dominant phenotype of a trait
• Mendel's Law of Independent Assortment

  "Each pair of alleles (gene) assorts independently of each other pair of alleles (gene) during gamete formation"
• Dihybrid cross

  A cross involving parents differing in two characters
• The F2 generation in Mendel's dihybrid crosses showed two new phenotypes in addition to the two parental phenotypes, in a 9:3:3:1 ratio
• If independent assortment did not occur, the F2 would have no recombinant phenotypes – only parentals
• Mendel proposed his Law of Independent Assortment to explain his observations
• Solving more complex genetics problems

  1. Deal with each of the 3 allele pairs separately

  2. Calculate the probability for each allele pair

  3. Multiply the probabilities together to give the overall probability
• Chromosome Theory of Inheritance

  Sutton wrote that the association of paternal and maternal chromosomes in pairs and their subsequent separation during the reducing division may constitute the physical basis of the Mendelian law of heredity
• Comparison of Mitosis and Meiosis


  • In mitosis there is only one cell division, in meiosis there are two
  • Synapsis is unique to meiosis
  • Meiosis produces 4 (non-identical) haploid cells (gametes), mitosis produces 2 identical diploid cells
  • Mitosis occurs in somatic cells, meiosis occurs in the germ line
  • The role of mitosis is to drive growth and tissue repair; the role of meiosis is to produce haploid gametes and to introduce genetic variability
• Homologous pair


  The two chromosomes of a homologous pair (or 'homologues') are individual chromosomes that were inherited from different parents
• Homologues


  Homologues appear alike under the microscope, but they have different versions of genes at some of their corresponding loci
• Chromatid


  One of the two identical strands of a newly replicated chromosome
• Sister chromatids

  Two identical chromatids held together by a common centromere following replication
• During interphase, note that chromosomes have not yet condensed
• Mendelian inheritance has its physical basis in the behaviour of chromosomes during meiosis
• Sutton made the link between the behaviour of chromosomes during meiosis and Mendel's Laws
• Chromosome Theory of Inheritance

  States that Mendel's 'heritable factors' (or genes) are located at specific positions (loci) on chromosomes, and it is the chromosomes that undergo segregation and independent assortment
• Gene

  Units of heredity that determine traits, located at particular loci on chromosomes
• Allele

  Different forms of a gene, one on each member of a pair of homologous chromosomes