Linkage and genetic mapping Flashcards
Genes on the same chromosome are inherited together (syntenic)
Linkage
Set of linked genes
-Genes located on chromosome 4 are in linkage group IV
Linkage group
Constructed based on the rate of recombination between linked genes using test crosses
-determine the order/arrangement of genes on a chromosome
Linkage map (genetic map)
Place on a homologous pair of chromosomes at which a physical exchange is occurring (site of cross over)
Chiasma
- The reciprocal exchange of chromatid segments at corresponding positions along homologous chromosomes (involves breakage and rejoining of 2 chromatids)
- leads to genetic recombination (different combination of alleles in the progeny)
Crossing over
Two extreme scenarios
- Independently assort
2. Genes are very close on the same chromosome
Genes are on different chromosomes or far apart on same chromosome
Independently assort
No recombination between genes
Genes very close on the same chromosome
Distance between genes (or cM for centiMorgan)
Map units (mu)
% recombination = #recombinants / Total # progeny
Map units calculation
Cross a triple heterozygote with a triple homozygous recessive
Mapping three genes using a three-point testcross
Occurs when two chiasmata form and there is a double exchange between two nonsister chromatids
Double Cross Over
Important to Remember!
The expected frequency of DCO gametes is much lower than that of either SCO gamete class.
-DCO is less likely to happen than single crossover
Important to Remember!
- Progeny in the lowest abundance result from DCO
- Progeny in the highest abundance is parental
Limits to Genetic maps
- DCO do not happen as often as predicted
- one chiasma inhibits formation of another nearby chiasma: interference
- Sometimes DCO can restore/maintain the parental phenotype
- Crossing over is not random; there are recombination hot and cold spots
