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 allele, lowercase for recessive
  • Codominant: base letter for gene, superscript for allele
  • Multiple alleles: can't use capital/lowercase, use base letter
  • Sex linkage: show X and Y chromosomes, allele only on X
  • Autosomal linkage: capital/lowercase for two different genes
• 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 within 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
• Genetic variation


  Differences in phenotype within a population due to genetic and environmental factors
• Sources of genetic variation

  • Mutations
  • Random fertilization of gametes
  • Meiotic crossing over and independent segregation
• Natural selection


  Organisms with advantageous phenotypes more likely to survive and pass on favourable alleles
• Disruptive selection


  Individuals with extreme traits more likely to survive, leading to loss of middling traits
• Speciation


  Creation of a new species due to reproductive isolation
• Allopatric speciation


  Geographical barrier separates populations
• Sympatric speciation


  Reproductive mechanisms prevent interbreeding in the same geographical area
• Genetic drift


  Change in allele frequency within a population due to chance events