Linkage and Chromosome Mapping Practice Problems (page 4)

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By — McGraw-Hill Professional
Updated on Aug 21, 2011

Practice 10

The cross (abc) × (+ + +) is made in an ascomycete with unordered tetrads. From the analysis of 100 asci, determine the linkage relationships between these three loci as completely as the data allow.

  1. 40 (abc)(abc)(+ + +)(+ + +)
  2. 42 (ab+)(ab+)(+ + c)(+ + c)
  3. 10 (a + c)(+ + c)(ab +)(+ b +)
  4. 8(a + +)(+ + +)(abc)(+ bc)

Solution 10

Pattern 1 is parental ditype (PD) for ab, ac, and bc. Pattern 2 is PD for ab, nonparental ditype (NPD) for ac and bc. Pattern 3 is tetratype (TT) for ab and ac, NPD for bc. Pattern 4, is TT for ab and ac, PD for bc. For each pair of markers the relative frequencies of each type of tetrad are as follows:

Solved Problems

For the ab pair, PDs are not equivalent with NPDs. Thus, a and b must be linked. For pairs ac and bc, PDs are roughly equivalent with NPDs. Thus, c must be assorting independently on another chromosome. The recombination frequency between a and b is

The recombination frequency in percent is equal to 9 map units. Single crossovers between either b and its centromere or c and its centromere, or both, would produce TTs for bc. Since none occurred we can assume that the loci of b and c are both very near their respective centromeres.

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