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BIOL 205
Ian D Chin- Sang

BIOL205:week 4, lecture 11 Longer regions have more crossovers and thus higher recombinant frequencies Three Point test cross Don't have veins on wings Cuts in wing Trihybrid females are test crossed with triple recessive males: → Test cross is usually done with tester male because Drosophila males do not show meiotic recombination. How many gametes from trihybrid? Use branch diagram parental Q. Why doesn’t the v and cv RF add up to 19.6%? (18.5%?) → assume 19.6% is more accurate: tend to take frequencies between two smaller frequencies • when we calculated the RF value for v and cv we did not count the v ct cv+ and v+ ct+ cv genotypes; after all, with regard to v and cv, they are parental combinations (v cv+ and v+ cv • this leads to an underestimation of the recombinant frequency. • normally this not a problem as the sum of the two shorter distances gives us the best estimate of overall distance Are the crossovers in adjacent chromosome regions independent events or does a crossover in one region affect the likelihood of there being a crossover in an adjacent region? • generally, crossovers inhibit each other somewhat in an interaction called interference • in some regions, there are never any observed double recombinants (complete interference). • interference values anywhere between between 0 (no interference) and 1 (complete) and are found in different regions of the chromosome and in different organisms Gene order can usually be deduced by inspection • without a recombinant frequency analysis • typically, for three linked genes, we have the eight genotypes at the following frequencies:m two at high frequency (Parental) two at intermediate frequency (Recombinants) two at a different intermediate frequency (Recombinants) two rare (Recombinants from double crossovers) • w/ 3 genes only three gene orders are possible, each with a different gene in the middle position • the gene in the middle is that it is the allele pair that has “flipped” position in the double- recombinant classes • deducing gene order by inspection Different gene orders give different double recombinants ct
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