Figure 1.

Genetic analysis of marker assortment in tetrads. (Top) Two homologous pairs of chromosomes and their four chromatids (1–4) are shown at metaphase of meiosis I. Spindle fiber attachments (vertical lines) to the centromeres (filled circles) result in the separation of homologous centromeres. Two genetic loci (A and B) with Columbia (upper case) and Landsberg (lower case) alleles are shown. Recombination events (X) frequently separate distal markers (such as A) from the centromere. (Bottom) The results of scoring DNA markers in each of the members of a tetrad (i–iv) reveal the genotype. COs and the orientation of homologs at meiosis I dictate whether marker pairs assort into one of three categories: parental ditype (PD), nonparental ditype (NPD), or TT. TT patterns result only when recombination between a markers and its centromere occurs (as in the example shown at Top).