Punnet squares
In order to work out the potential genotypes from two parents we utilise a technique known as punnet squares. Continuing with the tall/dwarf bean example from the genetics primer (and the origins of Mendelian genetics and punnet squares) we shall explore the different offspring possible from homozygous tall beans (HH), heterozygous tall beans (Hh) and dwarf beans (hh).
First, we choose our two parents. For the first example we'll take two heterozygous tall beans (both Hh). We place one parent on the top of the punnet square, and the other parent on the left side:
Place parents: |
First Parent |
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Second parent |
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The empty squares are called the child squares, which will tell us the genotypes of all the expected offspring.
We write each allele of the genotype for each parent in a seperate box, e.g. First parent Hh, so write the allele 'H' in the first box below the parent, and the allele 'h' in the second box.
Write alleles: |
First Parent |
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H |
h |
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Second parent |
H |
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h |
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Note how the dominant allele is always written first, and the recessive allele is written second.
To find out the combination of offspring, we complete each square by taking the allele of the first parent that matches that column, and the allele of the second parent that matches that row:
First child square: |
First Parent |
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H |
h |
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Second parent |
H |
HH |
|
h |
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Second child square: |
First Parent |
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H |
h |
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Second parent |
H |
HH |
Hh |
h |
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Third child square: |
First Parent |
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H |
h |
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Second parent |
H |
HH |
Hh |
h |
Hh |
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Forth child square: |
First Parent |
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H |
h |
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Second parent |
H |
HH |
Hh |
h |
Hh |
hh |
|
Now we can read our results off the completed table, simply looking at the 4 child squares we have arrived at:
First Parent |
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H |
h |
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Second parent |
H |
HH |
Hh |
h |
Hh |
hh |
|
We see there are three different combinations we have arrived at - one square with HH, two squares with Hh, and one square with hh. Each square represents what genotype 1/4 of the offspring will have, so 25%. This means that since HH is a combination only seen in one square, 25% of the offspring should in theory inherit this genotype from these parents. Hh is seen in two squares, which is a total of 50% of the offspring. Finally, hh is again 25% since it is in one square only.
Looking at the phenotypes, we note that both HH and Hh code for tall beans. Since these occur in 3 of the 4 child squares, 75% of the offspring will be tall beans, and the remaining 25% will be dwarf beans. Physically it would be impossible to tell the difference between the homozygous and heterozygous unless they were bred from.
Let's fill in another punnet square for a different combinations of parents, one heterozygous tall bean (Hh), and a dwarf bean (hh).
Add the parents: |
First Parent (tall) |
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H |
h |
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Second parent |
h |
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h |
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Fill in the child squares : |
First Parent (tall) |
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H |
h |
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Second parent |
h |
Hh |
hh |
h |
Hh |
hh |
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List the combinations: 2 squares Hh (50%), 2 squares hh (50%).
So for these parents we have 50% of the offspring being heterozygous tall (Hh), and 50% of the offspring being dwarf.