Ascobolus forms linear octads. In a cross between a WT pur+ strain (+) and a mutant pur- strain (m), the pur region sometimes forms heteroduplex DNA.

a) Draw the chromosomes from the cross above, paired in meiosis, after heteroduplex DNA has formed in the pur region but before correction of any mismatches. Clearly show the heteroduplex DNA and the alleles. Indicate what must occur next to lead to a 5:3, and the 2:6, ratio ascus. If repair needs to occur, clearly indicate which strands must be repaired at each mismatch, and what they would be repaired to, for each ascus type.

b) When heteroduplex DNA forms in the pur region, 10% of the time the heteroduplex DNA is not repaired, 50% of the time it is repaired to pur+ (+), and 40% of the time it is repaired to pur- (m). What proportion of abnormal-ratio asci will be 6:2, 2:6, 3:l:l:3, 5:3, and 3:5? dr