IMED2004 Study Guide - Final Guide: Centimorgan, Meiosis, Genetic Distance
IMED2004 Notes: Crossing Over Location
Determining the location of crossover
● Know correct order and have already identified the NR and double crossover
progeny
○ Other four include progeny underwent single crossover event – 2
between st and ss & 2 between ss and e.
■ Compare alleles in NR with single crossover progeny
Calculate recombination frequencies
● Can determine map distances based on frequencies of recombination.
○ To determine accurate map distance, must include single AND double
crossover events.
■ st-ss recombination freq:
● (50+52+5+3)/755 x 100% = 14.6%
■ ss-e recombination freq:
● (43+41+5+3)/755 x 100% = 12.2%
Effect of multiple crossovers
● So far examined double crossover between 2 of 4 chromatids.
○ Called 2 strand crossover.
● Double crossover of 3 or 4 chromatids can also take place.
● Examine only alleles at loci on either side of double crossover:
○ 2-strand, no new combination (no R gametes)
○ 3-strand, 2/4 R gametes
○ 4-strand, all 4 R gametes
● All types of double crossover produce average of 50% R progeny
○ 3 or 4 strand double cross overs can occur during meiosis
Document Summary
Know correct order and have already identified the nr and double crossover progeny. Other four include progeny underwent single crossover event 2 between st and ss & 2 between ss and e. Compare alleles in nr with single crossover progeny. Can determine map distances based on frequencies of recombination. To determine accurate map distance, must include single and double crossover events. So far examined double crossover between 2 of 4 chromatids. Double crossover of 3 or 4 chromatids can also take place. Examine only alleles at loci on either side of double crossover: 2-strand, no new combination (no r gametes) All types of double crossover produce average of 50% r progeny. 3 or 4 strand double cross overs can occur during meiosis. Although we have 3 recombinant products, only half of them detected because if it occurs between loci not detected. 3 out of 4 are recombinant but only half detected.
