LECTURE 4: Linkage and Recombination

• Complete linkage 


  No recombination and want to get all parental types in the progeny of test cross
• Incomplete linkage


  there is recombination and it shows how to get recombination frequencies
• Special type


  Sex linkage (X linked, Y linked)
• Test cross 


  An experimental cross, e.g. Dominant phenotype (PP or Pp) x recessive phenotype (pp)
• independent assortment 


  If you have _ then the genes are located at different chromosomes (1:1:1:1)
• Parentals >50% & Recombinants <50%

  Genes are located on the same chromosome. The genes are linked
• Linkage


  linear arrangement of non-allelic genes (different locations on a chromosome) on the same chromosome ● genes do not assort independently ● genes can be separated by crossing-over (happens at pachynema)
• complete linkage


  Link genes are always transmitted together but that is more true if you have
• There are cases when genes even though they are located at the same chromosome they are not transmitted together it will happen when crossing over took place
• linkage 


  Three genes located at the same chromosome therefore we can say that these three are linked and do not assort independently and can only separated thru crossing over
• Independent Assortment


  located on different homologous chromosomes, one gene pair is assigned to a pair of homologous chromosomes
• ● Complete linkage


  The genes are very close to each other thus transmitted together. - all parental types (100%) - no recombinant types (0%)
• Incomplete linkage


  ● the genes are far from each other thus crossing-over is possible
• Incomplete linkage 


  ● recombinant types are obtained through crossing-over
• Incomplete linkage 


  ○ parental types >50% ○ recombinant types <50% (higher chance if higher frequencies of crossing over)
• Linkage group 


  ● physical association of genes on a chromosome ● number of linkage group is equal to the n (haploid no.)
• Linkage in Drosophila


  ● published by Thomas Hunt Morgan in 1910
• Linkage in Drosophilia


  ● X-linked traits in Drosophila ● confirmed the Chromosome Theory
• Alfred Sturtevant, Herman Muller and Calvin Bridges

  Who are the students of Morgan in 1915 that proposed the phenomenon of crossing-over, they also used linkage as tool for chromosome mapping?
• X-linked mutants

  ● % recombination is used as a measurement of distance (How frequent recombination takes place between them)
• X-linked mutants

  one map unit or 1 cM (centimorgan) is the distance that gives 1% recombination
• closer the gene

  more difficult to break the linkage (lower frequency recombination)
• 1. Make a three-point test cross 2.Establish the proper gene order 3.Compute for the distance between genes – based on % recombination 4. Calculate for cc (coefficient of coincidence) 5. Solve for interference

  What are the steps in gene mapping?
• ((SCO+DCO)/n)*100


  formula for getting the % cross over at regions (COi)
• Strength of linkage 


  measured by coefficient of coincidence (cc = highest value - 1, lowest value - 0 same with interference)
• cc=ADCO/EDCO = (DCO/Total Progeny)/(COi)(COii)

  formula for getting cc using adco and edco
• (i) = 1 – cc

  formula for interference
• i = 1

  there is complete interference (DCO are occurring as expected)
• i = 0

  there is no interference (DCO are not occurring as expected)
• i > 0.5

  near
• i < 0.5

  far
• the closer the genes are, the lesser is the chance that crossing over will take place between them
• Sex Linkage

  sex chromosomes carry other genes aside from genes for sex determination
• Sex Linkage

  • Genes are carried by sex chromosomes are not actually related to sex
• X-linked Inheritance

  can be due to sex-linked recessive genes (gene is carried by the x chromosome)
• examples of x-linked inheritance

  colorblindness - hemophilia - absence of central incisors - congenital deafness - congenital cataract (could be mother to son transmission)
• Y-linked inheritance

  - genes on the Y chromosome - father to son transmission
• Y-linked Inheritance

  shows holandric transmission
• webbing of the toes ( 2nd and 3rd toes are attached) , hypertrichosis

  examples of y-linked inheritance