What it shows
When two hybrid tall pea plants are crossed, each offspring gets one gene copy from each parent at random. Tossing two coins 100 times copies this and shows the 1 to 2 to 1 genotype and 3 to 1 appearance ratios.
The science
In peas, tall (T) is dominant over short (t). A hybrid plant Tt carries one copy of each, and makes gametes with T or t in equal numbers. In a cross Tt × Tt, a coin toss models this: heads is T and tails is t for each parent. The offspring can be TT, Tt or tt in the ratio 1:2:1, so three quarters look tall and one quarter short. The ratio appears only when numbers are large, because chance causes scatter in small samples.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two coins of the same kind
- A table to record the tosses
- A pen or pencil
- A calculator
- Graph paper
- A group of classmates to pool results
- A chart of the cross written as a Punnett square
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Decide that each coin stands for one parent and heads means the tall allele T and tails means the short allele t.
Draw a table with columns TT (two heads), Tt (one head and one tail) and tt (two tails), and a tally column for each.
Toss both coins together, read the result as the offspring’s genotype, and make a tally mark.
Repeat 100 times and count the tally marks in each column.
Write the genotype ratio and combine TT and Tt as "tall" and tt as "short" to find the appearance ratio.
Pool your results with other groups to reach 400 or more tosses and compare the ratio with your own smaller sample.
Draw the Punnett square for Tt × Tt and compare it with the result of the tosses.
Write why the pooled ratio is closer to 3 to 1 than a small sample, and how this connects to Mendel’s peas.
What you should see
Your 100 tosses give roughly 25 TT, 50 Tt and 25 tt, with some scatter, and about 75 tall to 25 short. The pooled results lie closer to the expected ratios.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does a dominant allele mean?
It shows its effect even when only one copy is present.
2What is the expected ratio of TT, Tt and tt?
1 to 2 to 1.
3Why do small samples not give perfect ratios?
Chance causes random variation, which shrinks with more trials.
4Who first worked out these laws using peas?
Gregor Mendel.
Take it further
Use two pairs of coins of different kinds for two traits, such as seed shape and colour, and find the 9 to 3 to 3 to 1 ratio for a dihybrid cross.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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