Meiosis and the origin of gamete variation
BiologyCell BiologyAges 15–16Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Cell Biology
Step through each phase of meiosis I and II in a cell with n pairs of homologous chromosomes. Toggle crossing over and watch the number of possible gamete types change: independent assortment alone already gives 2ⁿ types, and adding crossing over sends the number of combinations soaring — this is exactly why offspring differ from their parents. A Slide spotting screen shows stained onion anther and grasshopper testis cells from leptotene to telophase II: identify each stage, count the chromosomes per cell and put the twelve slides in order.
Lesson: Meiosis
What it shows
Meiosis halves the chromosome number: one diploid cell divides twice to give four haploid gametes. At metaphase I each homologous pair lines up independently, so independent assortment alone gives 2ⁿ gamete types, where n is the number of pairs; crossing over between non-sister chromatids in prophase I adds recombinant chromosomes and many more combinations. The Slide spotting screen trains the practical skill of recognising the stages, including the five sub-stages of prophase I, in stained onion anther and grasshopper testis squashes, and of counting chromosomes: 2n per cell up to anaphase I, n after telophase I.
How to use
Set the Number of homologous chromosome pairs (n), tick or untick Crossing over and step through with Next phase ▶ or Auto-play, comparing the number of gamete types. On the Slide spotting tab, choose a Specimen, pick the Stage, enter Chromosomes per cell and press Check for each slide; then switch the Task to Put in order and tap the twelve slides in sequence.
Parameters you can change
- Number of homologous chromosome pairs (n) 1–4
- Crossing over occurs
- Auto-advance through phases
- Screen Stages and variation, Slide spotting
- Specimen Onion flower bud (anther), Grasshopper testis
- Slide task Identify stages, Put in order
Questions to explore
- With n = 3 and no crossing over, how many different gamete types are possible?
- Between which chromatids does crossing over take place, and in which phase?
- In which division, meiosis I or meiosis II, is the chromosome number halved?
