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In plant and animal cells, DNA is kept in the nucleus, packed into chromosomes. A chromosome is a thread made of protein and one very long DNA molecule.

Human cells have 46 chromosomes in 23 pairs. One of each pair comes from each biological parent.

A gene is a section of DNA that is a basic unit of heredity. Many genes are the instructions for building a protein. Humans have about 20,000 genes.

Experiment: Coin-toss babies

1. Each coin is one parent. Both parents have two alleles for one gene: B and b. Heads means the parent passes on B, tails means b.
2. Predict: out of 20 'babies', how many will be BB, Bb and bb?
3. Toss both coins together. Write down the pair you get (heads + tails counts as Bb, in either order).
4. Repeat until you have 20 babies. Use the same coins and the same way of tossing each time: that keeps it fair.
5. Add up your tallies. Then add the results of the whole class. Which is closer to your prediction: your 20 tosses or the class total?

Baby's alleles	Tally	Total
BB	Predict the totals out of 20.
Bb	Predict the totals out of 20.
bb	Predict the totals out of 20.

Questions

1. Put these in order from largest to smallest: gene, chromosome, nucleus.

2. Write the partner strand for: A A T G C C.

3. Write your class's totals for BB, Bb and bb. Were they close to 1:2:1?

4. Why does tossing more coins make the result closer to the prediction?

Sources for every fact on this sheet: echoesof.science/school/dna/sources/

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