The bead experiment
You can watch a 1 : 2 : 1 ratio build up in an afternoon, with no plants at all. Two bags of beads behave the same way gametes do, for exactly the same reason.
You need
- Two opaque bags or boxes you cannot see into.
- Fifty beads of one colour and fifty of another, divided so that each bag holds twenty-five of each colour.
Method
- Shake each bag so the beads are thoroughly mixed.
- Without looking, take one bead from the first bag and one from the second.
- Record the pair of colours you drew.
- Return both beads to the bags they came from and shake again.
- Repeat until you have recorded a hundred pairs, then count how many of each combination came up.
What each part of the model stands for
| In the model | In the plant |
|---|---|
| One bag | One parent |
| One bead | One gamete, carrying a single factor |
| A pair drawn | One fertilisation — the two factors of one offspring |
| Returning the beads | The parent still has both factors to pass on to the next offspring |
What you should find
Roughly a quarter of your hundred pairs will be both of the first colour, roughly a quarter both of the second, and roughly half will be one of each — a 1 : 2 : 1 ratio. It will not be exact. Do another hundred draws and it will get closer.
Step 4 is not optional
If you keep the beads out, the bag changes as you go and later draws are no longer equally likely. A parent plant does not run out of factors when it makes a gamete, so the model has to put the bead back to stay honest.