AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

GENETIC MAP / LINKAGE MAP

• Each gene has definite order and location in a linkage group or chromosome.
• The percentage or frequency of crossing over appears to be directly proportional to physical distance between two given genes. So the genetic maps are also referred to as cross over map .
• Linkage map is a diagrammatic graphical representation of relative distances between linked genes in a chromosome.
• The percentage of crossing over is calculated by test crosses.
• If the percentage of crossing over between two linked genes is 1 per cent (yields 1 per cent recombinant chromosome or gametes), the map distance between these genes is one unit of map distance, which is known Map unit, Morgan unit or Centimorgan (cM).
  • Examples
    • If a F₁ hybrid having the genotype (Ab)(aB) produces 8% of “AB” and 8% of “ab” cross over gametes, then the distance between “A” and “B” is estimated to be 16 Map units or Morgan unit or Centimorgan.
    • If the map distance between the gene loci “C” and “D” is 10 centimorgan, then 10% of gametes of genotype (CD)(cd) should be cross over types, i.e., 5% “Cd” and 5% “cD”.
• After determining the relative distances between the genes of a linkage group, it becomes easy to place genes in their proper linear order.
• For example, five genes A, B, C, D and E are to be plotted on a chromosome. All the gene combination crossing over percentages are to be found. The two genes which have highest percentage of crossing over should be placed on each end.

  • In this example, genes A and E have the highest percentage of 30% of crossing over; it means that these should be placed at the maximum distance.
  • The gene A can be taken as a starting point in the chromosome and can be represented by 0.
  • The gene A and B exhibit 6% crossing over, the gene B can be plotted on the chromosome at a distance of 6 units.
  • The gene C shows 15% crossing over with gene A and 9% crossing over with gene B, it can be plotted on the chromosome at a distance of 9 units from gene B.
  • The gene D shows 25% crossing over with gene A and 10% crossing over with gene C, it can be plotted on the chromosome at a distance of 10 units from gene C.
  • The gene E shows 30% crossing over with gene A and 5% crossing over with gene D, it can be plotted on the chromosome at a distance of 5 units from gene D.
    

     

  To view "Genetic Mapping Animation"