Chromosome Linkage, Recombination, & Mapping
In Principle:

Loci linked on same chromosome do not assort independently
          Genetic ratios in gametes vary according to how close they are situated.

Chromosome segments are physically exchanged between chromatids during meiosis
       Chiasmata (sing., a chiasma) are cytological indications of cross-overs
       DNA strands are broken and undergo physical recombination

Recombination is proportional to distance between loci
       % recombination can be use to produce gene maps
       Tri-hybrid test crosses map three loci simultaneously

Recombination maps link classical to molecular genetics
Loci that occur on the sex chromosomes are sex-linked
       Sex-influenced or sex-limited traits are not necessarily sex-linked:
             e.g., Breast cancer in females is associated with an autosomal locus BRCA1
                      Male pattern baldness and poly-cystic ovaries may be alternative phenotypes
Recombination among loci occurs two ways
       Unlinked loci on separate chromosomes assort independently,
            according to Mendel's Third Law
       Linked loci on the same chromosome tend to assort together, < or << 50% [IG1 14.10]
          Recombination frequency is proportional to physical distance among loci [IG1 Res Brief 15.2, pp. 306-307]
            Crosses with cis & trans configurations yield different results
               Analysis of dihybrid cross: linked A & B loci (PDF notes)
               Analysis of trihybrid cross: linked E, F, & G loci (PDF notes) (Powerpoint demonstration)                  
                     Determination of recombination frequency: identify & count recombinant classes
                         1 Map unit (m.u.) = 1 centimorgan (cM) = 1% recombination
                     Correction for double-recombinant events
                          or determination of gene order by "switch" rule                    
                     Possibility of Interference
       Genetic maps bridge classical genetics and molecular biology [IG1 15.20,21]
                Linkage maps are co-linear with chromosomes
                Linkage maps for morphological phenotypes in Garden Peas, Drosophila, and Solanum
                Linked loci separated by >50 cM or on opposite sides of centromere show random recombination

Homework: Practice trihybrid cross examples [PDF copy]
All text material ©2016 by Steven M. Carr