Summary GTS161 study unit 6 notes

Theme 6
Linkage
• Genes on the same chromosome are said to be linked and demonstrate linkage in
gene4c crosses
• Chromosome mapping
§ determine the posi4on of a gene on a chromosome
§ determine the order, rela4ve posi4ons and distances between linked genes
• Gene4c map - maps % recombina4on
• recombinant gametes - gametes in lowest frequency or not in P genera4on
• recombina4on generated between loci on
§ same chromosome by crossing over (linked genes)
§ different chromosomes (unlinked genes) by independent assortment




• if A and B are very far apart on same chromosome - crossing over will occur in every
meiosis - gametes obtained in = frequency
• if A and B are closer together on same chromosome (incompletely linked) - crossing
over not every meiosis / percentage recombinant gametes less than 50%
• if A and B are very close together on same chromosome (completely linked) - no
crossing over occurs therefor no recombinant genes
Rules for crossover between two loci
• frequency of recombinant gametes is always HALF the frequency if crossing over
• Max recombinant genes is 50%
• 20% of recombinant gametes means crossing over occurred in 40% of tetrads
Coupling and Repulsion




AB/ab Ab/aB