The order of three genes and the centromere on one chromosome of Neurospora is centromere - x -y - z. A cross between + + + and x y z produced an ascus with the following ordered array of ascospores (only one member of each spore pair is shown): (++z) (+y z) (x + +) (x y +).
(a) Is this ascus most likely the result of a meiotic event in which 0, 1, 2, or 3 crossovers occurred?
(b) In what interval(s) did the crossover(s) most likely occur?
(c) If double or triple crossovers were involved, were they two-strand, three-strand, or four strand multiple crossovers?