Probability and Genetics How is probability related
to genetics? Think back to Mendel's
experiments. He carefully
counted the offspring from
every cross. When he crossed two
plants that were hybrid for stem
height (Tt), about three fourths
of
the F2 plants had tall stems. About
one fourth had short stems.
Each time Mendel repeated the cross, he observed similar
results. He realized that the principles of probability
applied
to his
work. He found that the probability of a hybrid cross
producing a
tall plant was 3 in 4. The probability of producing a short
plant
was 1 in 4. Mendel was the first scientist to recognize that
the
principles of probability can predict the results of genetic crosses.
Punnett Squares
A tool that can help you grasp how the laws of probability apply
to genetics is called a Punnett square. A Punnett square is a chart
that shows all the possible ways alleles can combine in a genetic
cross. Geneticists use Punnett squares to see these combinations
and to determine the probability of a particular outcome, or result.
In a genetic cross, the combination of alleles that parents
can pass to an offspring is based on probability.
Figure 1 shows how to make a Punnett square. In this case, the
cross is between two hybrid pea plants with round seeds (Rr). The
allele for round seeds (R) is dominant over the allele for wrinkled
seeds (r). Each parent can pass either one allele or the other to
an offspring. The boxes in the Punnett square show the possible
combinations of alleles that the offspring can inherit.
2 The male parent's
alleles are written along
the top of the square.
Fill in the female
parent's alleles along
the left side.
R
r
FIGURE 1
INTERACTIVE ART How to Make a
Punnett Square
You can use a Punnett square to find the
probabilities of a genetic cross.
Follow the steps in the figure to fill in
the
Punnett
square.
1. Predict What is the probability that an
offspring will
have wrinkled seeds?
1 Start by drawing
a box and dividing
it into four squares.
2. Interpret Tables What is the probability
that an offspring
will have round seeds?
Explain your
answer.
82 Genetics: The Science of Heredity