Lecture 4

• Linkage


  Linear arrangement of non-allelic genes on the same chromosome
• Linkage


  • Genes do not assort independently
  • Genes can be separated by crossing-over
• Types of Linkage

  • Complete Linkage
  • Incomplete Linkage
• Complete Linkage


  Genes are very close to each other, thus transmitted together
• Complete Linkage

  • All parental types (100%)
  • No recombinant types (0%)
• Incomplete Linkage


  Genes are far from each other, thus crossing-over is possible
• Incomplete Linkage


  • Parental types >50%
  • Recombinant types <50%
• Linkage Group


  Physical association of genes on a chromosome
• Linkage Group

  • Number of linkage groups is equal to the haploid number (n)
• Linkage in Drosophila was published by Thomas Hunt Morgan in 1910
• X-Linked Mutants


  • Percent recombination is used as a measurement of distance
  • One map unit or 1 cM (centimorgan) is the distance that gives 1% recombination
• Genetic or Linkage Map


  • Linear arrangement of non-allelic genes on the chromosome
  • Distance between genes is based on % recombination
• Coefficient of coincidence (cc)


  Measure of strength of linkage
• Interference (i)


  1 - cc
• If i = 1, there is complete interference
• If i = 0, there is no interference
• Sex linkage


  Sex chromosomes carry other genes aside from sex determination genes
• Examples of X-linked traits

  • Colorblindness, Hemophilia
• X-linked inheritance (recessive)


  mother to son transmission
• x-linked inheritance (dominant)

  father to daughter inheritance
• y-linked inheritance


  Genes on Y chromosome, father to son transmission
• y-linked traits

  Webbing of toes

  Hypertrichosis
• Genetic sex determination

  Specific genotypes

  Multiple alleles

  Multiple genes
• Environmental sex determination

  Sex determined by environmental factors
• Environmental sex determination


  • Marine worms

  Coral reef fish
• Chromosomal sex determination


  Association of sex characteristics with a particular chromosome
• XX-XY system: females have Barr body
• Barr body


  Inactive X chromosome, for dosage compensation
• Number of Barr bodies = Number of X - 1
• Lyonization or X-inactivation


  Process of X-inactivation discovered by Mary Lyon
• the greater the distance of gene, there is higher probability of crossing over and higher recombination frequency due to more space for chiasmata formation
• genetic recombination

  • production of new allele combination not found in the parents