Constructing a Punnett Square
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Completing the Punnett Square
Once you have decided on the gamete combinations you can then complete the Punnett Square.
Luckily for you this practical comes with a Punnett Square calculator (we will use it shortly), so all you have to do is correctly identify the gametes!
We will now have a look at the calculator...
Using the Punnett Square Calculator
You have a two Blue Brown Frogs, one brown male, and one blue female, which you decide to cross. These frogs are the P generation (parent generation), their offspring will be the F1 generation.
From previous work you know the male is homozygous for the brown gene (so is 'AA') and the female must be homozygous for blue as it is recessive, and so is 'aa'.
1) How would the alleles be split between the gametes in the male frog?
One answer only.
   100% of the gametes would contain 'A'
   100% of the gametes would contain 'a'
   50% of gametes would contain an 'a' and 50% an 'A'
2) How would the alleles be split between the gametes in the female frog?
One answer only.
   100% of the gametes would contain 'A'
   100% of the gametes would contain 'a'
   50% of gametes would contain an 'a' and 50% an 'A'
  1. In the Punnett Calculator below type in 'AA' (without the quotes) in the text box labelled 'Top:', this will represent the male frog (AA).
  2. In the Punnett Calculator below type in 'aa' (without the quotes) in the text box labelled 'Left:', this will represent the female frog (aa).
  3. Click calculate... and answer questions in the quiz!
Punnett Calculator
Left:
Top:
1) From your cross of the homozygous male brown Blue Brown Frog, with a blue female Blue Brown Frog (P generation), what percentage of the offspring (F1 generation) would be brown?
One answer only.
   0%
   10%
   20%
   25%
   40%
   50%
   75%
   100%
2) What percentage of the F1 generation from the above cross of the P generation would be homozygous for brown skin?
One answer only.
   0%
   10%
   20%
   25%
   40%
   50%
   75%
   100%
3) If you crossed a brown male with a brown female offspring from the F1 generation, what percentage of the offspring (F2 generation) would be blue (use the calculator above to work it out)?
One answer only.
   0%
   10%
   20%
   25%
   40%
   50%
   75%
   100%
Summary
We have now looked at how to construct a Punnett Square for a 1 gene cross (a monohybrid cross), and we have:
  1. seen how to construct a Punnett Square
  2. looked at how characteristics are passed from the parent generation (P generation) to the next (F1 generation)
  3. seen how a crosses of the F1 generation lead to characteristics in the second generation (F2 generation)
Next we will go on to look at using Punnett Squares for situations where we have two alleles - a dihybrid cross.
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