How to set up a Punnett Square – For a One-Factor Cross
How to set up a Punnett Square – For a One-Factor Cross
1. Determine what genotypes are being crossed (male and female).
1. Determine what genotypes are being crossed (male and female).
2. Write the cross – typically you write the male genotype first. Example:
(male genotype) X (female genotype)
Bb X BB
2. Write the cross – typically you write the male genotype first. Example:
(male genotype) X (female genotype)
Bb X BB
3. Draw a 2 by 2 grid, making 4 boxes.
3. Draw a 2 by 2 grid, making 4 boxes.
4. Write the male’s alleles (possible gametes) along the top of the grid.
Write the female’s alleles (possible gametes) along the side of the grid.
B
b
Male alleles
4. Write the male’s alleles (possible gametes) along the top of the grid.
Write the female’s alleles (possible gametes) along the side of the grid.
B
b
Male alleles
Female alleles
B
Female alleles
B
B
B
5. Fill in the boxes of the grid according to the corresponding male (top)
and female (side) alleles given.
B
b
5. Fill in the boxes of the grid according to the corresponding male (top)
and female (side) alleles given.
B
b
B
BB
Bb
B
BB
Bb
B
BB
Bb
B
BB
Bb
- Note: typically, if there both dominant and recessive alleles are present
(heterozygous), you write the dominant one first.
Example: Write Bb (NOT bB)
- Note: typically, if there both dominant and recessive alleles are present
(heterozygous), you write the dominant one first.
Example: Write Bb (NOT bB)
6. Each box shows the potential combination of alleles (the genotype) for
an offspring. In the case of a one-factor cross, each box represents a 1
in 4, or 25%, probability that the offspring will have that genotype.
6. Each box shows the potential combination of alleles (the genotype) for
an offspring. In the case of a one-factor cross, each box represents a 1
in 4, or 25%, probability that the offspring will have that genotype.
- In the example above, there is a 2 in 4 chance, or 50%, probability that
the offspring’s genotype will be BB. There is the same probability that the
offspring’s genotype will be Bb.
- In the example above, there is a 2 in 4 chance, or 50%, probability that
the offspring’s genotype will be BB. There is the same probability that the
offspring’s genotype will be Bb.