Gene Mapping, Crossing Over, Centromere Assignment

Lab 9: Gene Mapping, Crossing Over, Centromere Mapping (Sordaria fimicola) Homework assignment 1. a. 2 b. B and C c. (100 + 100 + 90 + 90)/(1680) x 100% = 22.6 map units 2. Although Sordaria are usually haploid, at a certain stage they sexually reproduce by fusing two haploid spores to form a diploid cell. This cell immediately undergoes Meiosis I, Meiosis II, and one round of Mitosis, two form an eight-cell haploid array. In haploid cells, the genotype always reflects the phenotype, and so easily visualized alleles (like spore color) can be determined. These two features allow us to study crossing over during ascus formation. 3. The first, second, fourth, and fifth asci are the result of second division segregants (i.e. where crossing over has occurred): [(30 + 74 + 65 + 43) / (780 x 2)] x 100% = 13.6 map units 4. The first, second, third, and fourth asci are the result of second division segregants (i.e. where crossing over has occurred): [(52 + 65 + 120 + 130) / (777 x 2)] x 100% = 23.6 map units 5. Homologous recombination occurs when, during the chiasma formation where two double-stranded chromosomes pair closely together, the double-stranded DNA of one chromosome is swapped with the double-stranded DNA of the other chromosome. This produces the typical second division segregant patterns in Sordaria. During chiasma formation, it can sometimes happen that a piece of single-stranded DNA can bind to a single-stranded DNA from a different chromosome. The cell tries to repair this by converting one strand into the sequence of the other. The upshot is that sister chromatids are no longer identical, which results in unusual patterns (you could think of it as third division segregants). ! 1