Meiosis & Crossing Over

Meiosis & Crossing Over is an interactive 3D cell-division simulation that demonstrates how sexual reproduction generates genetic diversity while halving chromosome number from diploid (2n) to haploid (n). Inside a translucent dividing germ cell with centrosome poles and spindle fibers, two pairs of homologous chromosomes (maternal chromatids in rose-red, paternal chromatids in sky-cyan) progress through Prophase I, Metaphase I, Anaphase I, Telophase I, Metaphase II, Anaphase II, and final Haploid Gametes. Students can adjust Crossing over frequency (or drag inner chromatids together) to swap distal allele segments at glowing chiasmata, toggle Metaphase I chromosome flip for independent assortment, and track the resulting four recombinant and parental gamete genotypes.

• In Stage 1 (Prophase I), set Crossing over frequency to 1 or 2 and observe the glowing yellow chiasma ring where inner non-sister chromatids exchange red and cyan allele tips.
• Advance to Stage 2 (Metaphase I) and toggle Chromosome flip (Assortment) to switch which pole the maternal and paternal homologues of Pair 2 face.
• Step through Stages 3 and 4 (Anaphase I and Telophase I) to see homologous pairs separate into two haploid (1n) cells while sister chromatids remain joined at their centromeres.
• Continue to Stages 6 and 7 (Anaphase II and Four Haploid Gametes) to see centromeres split and compare the four resulting recombinant and parental gametes.
• Compare the Unique gamete genotypes readout when Crossing over is set to 0 (2 gamete types) versus 1 or 2 (4 unique gamete genotypes).

1. 1Explain how Meiosis I (reduction division) separates homologous chromosome pairs to reduce ploidy from diploid (2n) to haploid (1n).
2. 2Describe how synapsis and crossing over at chiasmata during Prophase I create recombinant chromatids with new allele combinations.
3. 3Explain how independent assortment of homologous pairs on the Metaphase I plate produces 2^n possible chromosome combinations.
4. 4Contrast Anaphase I (separation of homologous chromosomes) with Anaphase II (splitting of centromeres and separation of sister chromatids).

Questions teachers ask