Principles of Genetics

• Gregor Mendel

  • Austrian monk who suggested two principles of inheritance:

  1. Hereditary characteristics are controlled by paired factors (genes)
  2. During meiosis each gamete receives one copy of each gene which recombine during fertilisation (the principle of independent assortment)
• Mendel experimented on peas to study inheritance

  • He started with plants pure breeding for a specific characteristics
  • i.e. all yellow seeds or green seeds
  • He them interbred contrasting traits
  • i.e. a purebred yellow seed crossed with a purebred green seed
  • He discovered that the offspring (progeny) only resembled one parent
• The progeny

  • The progeny are referred to as hybrids as they have the genes for both yellow and green seeds but only exhibit the characteristics (phenotype) of one.
  • The characteristic exhibited (yellow seed) id the dominant trait
  • The characteristic masked (green seeds) is the recessive trait
• Walter Sutton

  • American geneticist who continued Mendel’s work and in 1903 proposed that:

  1. Chromosomes (and genes) are found in pairs in both zygote and somatic cells
  2. Chromosomes (and genes) segregate during meiosis - one to each gamete
  3. Chromosomes (and genes) maintain individually and do not blend with other chromosomes.
• Sutton also proposed

  • Since number of characteristics is > number of chromosomes then there must be factors chromosomes that cause characteristics (genes)
  • There are genes in particular chromosomes that link together to make sets (gene linkage)
  • Chromosomes of a pair carry genes that control the same characteristics - homologous chromosomes
• Chromosome terminology

  • Chromosome - rod like structure in nucleus that carries genetic information
  • Gene - series of bases that determines a characteristic - found on a chromosome
  • Allele - alternate forms of a gene
  • Locus - position of gene on chromosome
• Punnet Squares

  • The dominant gene gets a capital letter   
  • The recessive gene gets a lower case letter
  • The alternative forms of the same gene (e.g. Y and y) are called alleles
  • In 2 out of the 3 gene combinations (genotypes) the genes are the same YY or yy
  • This is called homozygous
  • The remaining combination Yy
  • This is heterozygous
• Punnet Square Terminology

  • Genotype - the genes that code for a characteristic
  • Phenotype - the characteristic exhibited
  • 1st Filial Generation (F1) - offspring of first monohybrid cross
  • 2nd filial generation (F2) -  offspring of F1 cross