Figure 1. Cartoon illustrating the generation of homologous pairs of chromosomes (bivalents) held together by chiasmata following crossing-over at pachytene. Two chromosome pairs are illustrated: one acrocentric, the other metacentric. Centromere positions are indicated by filled circles. One homolog of each pair is dark, the other light. At the end of pachytene, intimate homologous chromosome pairing (synapsis) lapses and the two homologs are held together only at the positions of crossing-over (chiasmata). The acrocentric pair, with one chiasma, forms a rod-shaped structure; the metacentric pair, with two chiasmata, forms a ring structure.

Figure 2. Human metaphase I spermatocyte. (a) Intact cell containing 22 pairs of autosomal bivalents and the XY bivalent. (b) Individual bivalents have been cut out and the positions of chiasmata are indicated by arrows.

Figure 3. Cartoon illustrating the relationship of a crossover, as viewed directly (as a chiasma or MLH1 focus) or indirectly in the products of meiosis (gametes).

Figure 4. Comparison of chiasma distributions (in 5% length intervals) along chromosomes 15 and 16. Chromosome 15 is acrocentric, chromosome 16 metacentric. Although both chromosomes are similar in length, they show different distributions.

Figure 5. Pachytene male (a) and female (b) germ cells. In both, synaptonemal complexes are stained green and MLH1 foci red. In the spermatocyte (a), the centromeres are stained blue. The chromatin cloud that surrounds the synaptonemal complexes has been omitted to aid clarity. Arrowheads indicate the MLH1 foci.

Figure 6. Comparison of the distribution patterns of MLH1 foci (in 5% length intervals) of chromosome 21 with one or two foci in fetal oocytes.

Figure 7. Histograms illustrating the distributions of chiasmata in spermatocytes (blue) and MLH1 foci in oocytes (pink) on chromosomes 13, 18 and 21. Each chromosome pair is divided into 5% length intervals. These crossover distribution patterns are also displayed for each chromosome as recombination maps. These maps highlight the different patterns of crossover numbers and distributions in male and female germ cells and the consequent effect on the recombination map.