2 TOPIC

• Sexual reproduction


  Formation of offspring that are genetically distinct from their parents
• Types of sexual reproduction in eukaryotes

  • Meiosis
  • Fertilization
• Types of sex determination based on phenotype

  • Hermaphroditism
  • Monoecious
  • Dioecious
• Ways sex may be determined

  • Chromosomally
  • Genetically
  • Environmentally
• Heterogametic sex


  Male produces two different types of gametes
• Homogametic sex


  Female produces gametes that are the same
• XX-XO sex determination


  • Females-XX
  • Males-XO
  • "O" signifies the absence of a sex chromosome
• XX-XY sex determination


  • XX-female
  • XY-male
  • Y chromosome is acrocentric, not Y shaped
• Pseudoautosomal regions


  Chromosomes that can pair because they are homologous in this small region, carrying the same genes
• ZZ-ZW sex determination


  • Female is heterogametic
  • Male is homogametic
  • Sex chromosomes are called Z and W
• Genic sex determination


  Sex is genetically determined, no distinct sex chromosomes, sex determined by genes on undifferentiated chromosomes
• Environmental sex determination

  Sex determined by environmental factors
• Sex determination in humans

  • XX-XY system
  • Presence of SRY gene on Y chromosome determines maleness
• Humans' sexual development is complex, influenced by SRY gene and other genes
• Most male and female secondary sex characteristics are encoded on autosomes, not sex chromosomes
• Dominance


  Phenotype of the heterozygote is the same as the phenotype of one of the homozygotes
• Incomplete dominance


  Phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes
• Codominance


  Phenotype of the heterozygote includes the phenotypes of both homozygotes
• Complete dominance


  • Heterozygote falls between phenotypes of two homozygotes, 3:1 phenotypic ratio in progeny
• Incomplete dominance


  • Heterozygote has intermediate phenotype, 1:2:1 phenotypic ratio in progeny
• Codominance


  • Heterozygote expresses both homozygote phenotypes
• Dominance is allelic interaction, does not alter inheritance but influences phenotype
• Penetrance


  Percentage of individuals with a genotype that express the associated phenotype
• Expressivity


  Degree to which a trait is expressed
• Incomplete penetrance and variable expressivity due to effects of other genes and environmental factors
• Lethal alleles


  Cause death at early development, modifying progeny ratios
• Recessive lethal alleles


  • Cause death only in homozygotes, 2:1 progeny ratio
• Dominant lethal alleles


  • Cause death in homozygotes and heterozygotes
• Multiple alleles


  More than two alleles present at a locus
• Number of possible genotypes with multiple alleles = [n(n+1)]/2, where n is number of alleles
• ABO blood group alleles

  • IA
  • IB
  • i
• Dominance relations of ABO alleles

  IA > i, IB > i, IA = IB
• Presence of antibodies against foreign ABO antigens allows compatible blood transfusions
• Mendel's principle of independent assortment - genes at each locus are independent in assortment and phenotypic expression
• Genotypes


  The genetic makeup of an organism
• Phenotypes


  The observable physical or biochemical characteristics of an organism
• ABO Blood Group


  • IA = encodes for the A antigen
  • IB = encodes for the B antigen
  • i = encodes for no antigen (O)
• Dominance relations among the ABO alleles

  IA > i, IB > i, IA = IB
• Antibodies are produced against any foreign antigens
• The presence of antibodies against foreign ABO antigens means that successful blood transfusions are possible only between persons with certain compatible blood types