From Seed to Seed:
Plant Science for K-8 Educators

Charting the Results

Let's fast-forward to the present, when we know about the existence of chromosomes and genes. You've probably guessed that Mendel's "hereditary factors" are now called genes. Following convention, we'll represent a dominant gene with an uppercase letter, and a recessive gene with the corresponding lowercase letter.

Let's go back to the peas, and the results of the seed color experiment. We will represent the gene for yellow color, which is dominant, with Y, and the gene for green color, which is recessive, with y.

Table 1.

Geneticists use a chart called a Punnett Square to show the outcome of genetic combinations. On the top and left side of the square are the genes contained by the two parents. Since these are pure parents, whose offspring consistently express one seed color over successive generations, we can assume that the yellow-seeded parents contain two dominant genes (YY) and the green-seeded parents contain two recessive genes (yy).

Table 2. F₁ generation

Now let's complete the chart by combining each of the letters across the top with each of the letters on the side. But first, let's look at the possible combinations when both the dominant and recessive genes are present. The possible combinations are YY, Yy, yY, and yy. And since Y is dominant, any plant that contains a Y gene will have yellow seeds: YY, Yy, yY. The only combination for which the recessive trait, green seeds, will be expressed is yy.

Table 3. F₁ generation

You can see that all of the offspring created by crossing these two pure parents contain the genes Y and y. Therefore, since they all contain Y, they all will have yellow seeds.

Next, let's see what happens when plants with this configuration of genes are self-pollinated.

Table 4. Self-pollinated F₁

F₁ generation: all Yy

F₂ generation: 1 YY, 2 Yy, 1 yy

You can see from the chart that, on average, three out of four seeds will be yellow, and one will be green. This is consistent with Mendel's observations.

Remember that these charts are used to predict the likelihood of a particular outcome. For example, you can say that there is a 25 percent chance that a seed produced by this F₂ plant will be green. In small samples, the percentages may be way off-these percentages reflect the probable results over a large sampling. Your students (grades 5-8) might enjoy creating Punnett squares for other crosses, and seeing what results they get!