Part II

1. The average of two, three, and four strand double-crossovers is fifty percent recombination between two linked loci. The expected ratio of two, three, and four strand double-crossovers is a 1:2:1 ratio, respectively.
   
   
2. When loci are linked, the parental or non-recombinant classes are the most frequent. The double-crossover classes are the least frequent type. The locus order is determined from three-point testcross data. The locus that is recombined in the least frequent class (double crossover) is the locus in the middle of the order of the three loci.
   
   
3. The reciprocal products that result from a crossover occur with approximately equal frequency. Differences between the number of reciprocal products is due to the sampling of gametes that produce the observed genotypes.
   
   
4. When calculating the recombination fraction between two loci, the double-crossover types must be included.
   
   
5. Interference reduces the number of double crossovers because the formation of one chiasma reduces the chances of a second chiasma forming close to the first. Interference exists when a single crossover in one region reduces the probability of a second single crossover in an adjacent region. If there was no interference, the probability of a double-crossover would be the product of the probability of a crossover in one region times the probability of a crossover in the second region. A negative interference means that the observed relative frequency of double crossovers is more than expected. A positive interference means that the observed relative frequency of double crossovers is less than expected.
   
   
6. The coefficient of coincidence is the observed number of double-crossovers divided by the expected number of double-crossovers. The interference is one minus the coefficient of coincidence.
   
   
7. For short chromosome segments, which are less than 10cM, map distances are additive because the likelihood of double crossovers is negligible. When the recombination fraction is greater than 10%, recombination fractions are no longer additive and must be converted to map distances using a mapping function.
   
   
8. 1-2r_AC = (1-2r_AB)(1-2r_BC). This equation shows that the probability of no recombination between the flanking markers A and C is equal to the probability of recombination between markers A and B times the probability of no recombination between markers B and C. If recombination fractions were additive then r_AC should equal r_AB + r_BC.