Figure 1. Some structures of mating-type loci. (a) Saccharomyces cerevisiae. Genes a1 and 1 are necessary for haploid mating functions. The protein products of a1 and 2 form a dimer that represses haploid functions; a2 has no known function. The differently coloured a and sequences are unrelated and different in length, though at the same chromosome locus. Flanking the expressed mating-type locus there are two (‘cassette’) loci (not shown), with ‘silent’ copies of a and respectively, that donate information in the a 2 switching process. (b) Neurospora crassa. The two mating types, A and a, are determined by two unrelated alternative segments at the same locus (idiomorphs, A containing three genes and a one). The protein products of genes A1 and a1 cooperate to bring about mating. A1 is also a heterokaryon-incompatibility gene. A2 and A3 provide postfertilization functions in fruit body development. Podospora anserina is very similar. Information from Glass and Nelson (1994). (c) Ustilago maydis. There are two mating-type loci, a and b. The a locus has two alternatives, a1 and a2, responsible for mutual recognition: pra1 encodes a receptor for mfa2-encoded pheromone, and pra2 a receptor for mfa1-encoded pheromone. The b locus has numerous alternative forms, two shown here, each with divergently transcribed genes of the E and W classes. Dikaryon development following cell fusion requires dimerization of E and W protein products, but only E–W combinations from different mating types are compatible: e.g. E1–W2 or E2–W1 but not E1–W1 or E2–W2. Information from Kämper et al. (1994). (d) Coprinus cinereus. Two loci, A and B, each present in the population in many different forms. The A locus (an idealized general representative shown here) contains paired and divergently transcribed HD1 (a₁, b₁, c₁) and HD2 (a₂, b₂, c₂) genes, analogous to the E and W genes of Ustilago, but in multiple copies with substantial sequence differences both within and between mating types. Stable dikaryon formation requires dimerization of an HD1 with an HD2 product, but HD1 and HD2 from the same mating type are mutually incompatible. The relatively widely separate and subloci show about 0.1% recombination. The B locus, analogous to Ustilago a, includes pheromones and pheromone receptor genes (respectively green and blue), but in multiple copies. Mutual recognition does not occur between pheromones and receptors encoded in the same mating type. Information from Casselton and Kües (1994) and O'Shea et al. (1998)

Figure 2. Segregation at (a) the first or (b) second division of meiosis of a pale-ascospore (e.g. yellow, y) mutation depending on whether or not there is crossing-over between the gene locus and the centromere. On the right are shown first- and second-division patterns seen in the asci. Since the orientation of disjoining centromeres is random with respect to the top and base of the ascus, the alternative spore arrangements indicated by the arrows are all equally likely.

Figure 3. The explanations in terms of crossing-over (absent, single or double) of (1) parental ditype, (2) tetratype and (3) nonparental ditype tetrads from crosses involving two linked markers. The example shown is the cross between two distinguishable pale-ascospore mutants of Sordaria brevicollis, buff (b) and yellow (y); the double mutant by is white. Double crossovers are infrequent, so the great majority of recombinants occur in tetratype asci. No account is taken here of first- versus second-division segregation, and the order of the spore pairs shown in the asci is arbitrary.

Figure 4. Mitotic crossing-over in an Aspergillus nidulans diploid heterozygous for recessive mutations paba (p-aminobenzoic acid requirement) and y (yellow rather than wild-type green conidia), linked in the same chromosome arm, with y the more distant from the centromere. Yellow-conidial sectors of mycelium result from mitotic crossing-over between y and the centromere when the chromatids disjoin 1, 3/2, 4 (50% probability). A yellow sector is also paba-requiring when the crossover is closer to the centromere than paba, but not when it is between the two loci. This example from Pontecorvo and Käfer (1958).