Lesson Genetics 101

Introduction to Genetics

This lesson is designed to assist in comprehending Gregor Mendel’s pea breeding experiments by playing with blocks!

Mendel cross-bred two different varieties of a species of pea — ‘yellow’ and ‘green’. He crossed the green with the yellow and noticed that all the offspring were yellow. When he planted seeds of these offspring so they would in turn interbreed, he found a 3:1 ratio of yellow to green plants. How did Mendel make sense of this outcome?

Set aside thoughts of vegetation and experiment with mathematical combinations of blocks. 4 blocks, 2 for yellow, 2 for green, enable us to follow Mendel’s reasoning.

Mendel’s Pea Breeding Experiment

The diagram below symbolizes Mendel’s ultimate interpretation of the evidence hidden in the first cross plants in his garden. We combine every block on the left hand side with every block on the right. 

This first cross gives an equal proportion of green and yellow blocks – even though all the cross bred plants themselves appeared to be yellow. To account for all the first cross outwardly appearing to be yellow, assume that the yellow blocks dominate the green. You can see that this means that all these 4 combinations will be yellow.

We now proceed to Mendel’s second cross, on the assumption that the ‘green’ and ‘yellow’ characteristics, as carried in the plants of the first cross, are represented by pairs of green and yellow blocks.

The diagram below symbolizes Mendel’s second cross in which he employed the seeds resulting from his first cross – all outwardly appearing to be yellow! Once again, every block on the left hand side is combined with every block on the right, symbolizing cross breeding.

This summary of all possible combinations reveals that three ‘yellow’ plants and one ‘green’ plant will be produced. Whenever a combination has a yellow block, that block is assumed to dominate the green one, resulting in a ‘yellow’ plant.

Do you find it surprising that you are able to decipher something about plants by using mathematical concepts? Why, when crossing ‘green’ and ‘yellow’ peas, was the outcome not simply a mixture of green and yellow – i.e. a different colour? As Mendel deduced, something must be involved in the make-up of living things that carries information in discrete packages.


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