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.
| pure
parents |
F1
generation |
F2
generation |
yellow seeds x
green seeds |
100% yellow seeds |
75% yellow seeds
25% green seeds |
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
yellow seeds |
| green
seeds
|
|
Y |
Y |
| y |
|
|
| y |
|
|
Now let’s complete the chart by connecting 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, in 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
yellow seeds |
| green
seeds
|
|
Y |
Y |
| y |
Yy |
Yy |
| y |
Yy |
Yy |
You can see that all 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
yellow seeds |
| yellow
seeds
|
|
Y |
y |
| Y |
YY |
Yy |
| y |
Yy |
yy |
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% 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. You
might enjoy creating Punnett squares for other crosses, and see what results you get!
