Determining two-factor distances.
Distinguishing linked from non linked genes by looking for double mutant animals
We define the recombination frequency (RF or p) as the percent of the recombinant products of the total.
Map distance is calculated in map units where 1 map unit (m.u.) = 1% recombination products = 1 centiMorgan (cM)
The trans Punnett Square
The cis Punnett Square
Using the trans heterozygote to make a double mutant
Problems (again seen when more and more genes are identified)
The relative position of genes close to each other cannot be determined by two-factor data.
When genes are linked but distant, multiple recombination
The assumption is that map distance is linear but it may not be
Solves the problem of the relative position of genes.
Textbook approach has flaws (see handout)
An alternative approach that uses only the existing strains ab/ab and c/c for which the distance between a and b is known (p).
Disadvantages or problems
These and other mapping procedures generate a series of maps for genes that are said to be on the same linkage group
Chromosome is the cytological manifestation
Linkage group is the genetically-derived operation definition
Bridges' experiment tied them together
Translocations and “Pseudolinkage”
Combining translocations with slight differences can produce organisms that are effectively deleted for various regions of the genome.
Robertsonian translocations in mammals
Speciation also involves inversions and translocations