Topic 7

• Genotype


  The genetic constitution of an organism
• Phenotype


  The expression of the genes but also the interaction with the environment
• Homozygous


  When you have a pair of homologous chromosomes carrying the same alleles for a single gene
• Heterozygous


  When you have the homologous chromosomes carrying two different alleles for a single gene
• Recessive allele


  Only expressed if there's no dominant allele present
• Dominant allele


  Always expressed
• Codominant


  Both alleles are equally dominant and expressed in the phenotype
• Multiple alleles


  More than two alleles for a single gene
• Sex linkage


  The gene whose locus is on the X chromosome
• Autosomal linkage


  Genes located on the same chromosome, not the sex chromosomes
• Epistasis


  One gene modifies or masks the expression of a different gene at a locus
• Monohybrid


  Inheritance of just one gene
• Dihybrid


  Inheritance of two genes at a time
• A genetic coding table is provided to help with different types of inheritance
• Genetic coding table examples

  • Monohybrid: capital letter for dominant, lowercase for recessive

  Codominant: base letter for gene, superscript for allele

  Multiple alleles: can't use capital/lowercase

  Sex linkage: allele only on X chromosome

  Autosomal linkage: two different letters for two genes
• Codominant example: Cow coat color


  Determine parental genotypes

  Use Punnett square to calculate probability of red offspring
• Epistasis example: Labrador coat color


  Determine parental genotypes

  Use Punnett square to work out all possible offspring phenotypes
• Dihybrid example: Mendel's pea plants

  Determine parental phenotypes and genotypes

  Work out gametes

  Use Punnett square to determine offspring genotypes and phenotypes

  Calculate phenotypic ratio
• Crossing over results in new combinations of alleles in the gametes, affecting the predicted Punnett square ratios
• Crossing over


  Results in new combinations of alleles in the gametes
• Autosomal linkage


  Two genes are located on the same chromosome, but not the X or Y chromosome
• Autosomal linkage


  1. Alleles for each gene are linked on the same chromosome

  2. Have to be inherited together

  3. Whole chromosome pulled to create one gamete

  4. Other chromosome pulled to create other gamete
• Autosomal linkage


  Only two types of gametes possible: dominant alleles or recessive alleles
• Autosomal linkage


  Results in a 3:1 ratio instead of 9:3:3:1
• Crossing over


  Creates new combinations of gametes
• Chi-squared


  Statistic used to investigate differences between expected and observed frequencies
• Using chi-squared


  1. State null hypothesis

  2. Convert ratio to expected frequency

  3. Calculate chi-squared value

  4. Compare to critical value

  5. Determine if significant difference
• Hardy-Weinberg principle


  Mathematical model to predict allele frequencies in a population
• Gene pool


  All the alleles of all the genes within a population at one time
• Population


  All the individuals of one species in one area at one time
• Adult frequency


  Proportion of an allele within a gene pool
• p


  Frequency of dominant allele
• q


  Frequency of recessive allele
• p^2


  Frequency of homozygous dominant genotype
• 2pq


  Frequency of heterozygous genotype
• q^2


  Frequency of homozygous recessive genotype
• Using Hardy-Weinberg equations


  1. Identify known values

  2. Calculate p and q

  3. Calculate other components
• Genetic variation


  • Caused by mutations, random fertilization, and meiotic processes
  • Leads to natural selection
• Disruptive selection


  • Individuals with extreme traits more likely to survive and reproduce
  • Middling trait lost over generations
• Speciation


  Creation of a new species due to reproductive isolation