Chance and offspring.

Step 1 of 6

An expected number

A Punnett square does not say what will happen, but what the chance is. Each box is a quarter of the chance, and each seed is a draw of one box. That is just what you learned in the probability Unit: equally likely outcomes, and an event that is some of them.

When there are many seeds, the chance becomes an expected number: the probability times the number of seeds.

Step 2 of 6

Mendel’s count

Mendel knew nothing of genes. He only counted, and in numbers big enough for the ratio to stand out from the noise. In $1866$⁠ he published his results, and almost nobody noticed. Only in $1900$⁠ did three researchers, each on their own, rediscover the same laws, and find that Mendel had beaten them by thirty-four years.

Step 3 of 6

Small samples

Now the bed has only four seeds. Cross the parents is fixed until you choose an answer, and then the bed will be sown and counted.

The chart beside the Scene shows, as the count goes on, the share of purple flowers among those counted so far, against the $75\%$⁠ line. In a small bed it jumps up and down.

Step 4 of 6

No memory

Change Seedlings to $100$⁠ and then to $929$⁠, and watch the chart: the more seedlings, the more closely the purple share hugs $75\%$⁠. But each seedling on its own is still a draw of a quarter against three quarters.

Step 5 of 6

Both at once

Sometimes the question is about two offspring together. The rule from the probability Unit works here too: when independent events must all happen, multiply their chances.

Step 6 of 6

Another cross

Every cross needs its own square. “A quarter” is right only for a cross of two heterozygotes.