Constructing a Punnett Square - Gametes
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Punnett Square
A Punnett Square is essentially a way to 'calculate' the outcome of haploid gametes combining to to form a diploid organism.
Let's look back at the Blue Brown Frog in which allele 'A' is dominant (it is a capital!) and causes the frog to have brown skin, and the 'a' is recessive and produces an animal with blue skin.
1) What would be the outcome of crossing a brown skinned Blue Brown Frog with a brown skinned Blue Brown Frog?
One answer only.
   a) All brown skinned frogs
   b) A mixture of blue and brown skinned frogs
   c) All blue skinned frogs
   a. or b.
   a., b. and c.
Did you get the above quiz right? It is quite difficult to decide the outcome of a cross between two organisms, and this is where Punnett Squares come in.
Two Punnett Squares are shown in Figure 1 below, and Punnett Square 1 could have been quite helpful in answering the above quiz as it shows a Punnett Square for a situation similar to the above question on the Blue Brown Frog.
Figure 1: Two Punnett Squares - Punnett Square 1 shows a simple 'one allele' situation and Square 2 shows two alleles - The parents for Punnett Square 1 would have been AA and Aa, and for square 2, Aabb and AaBB
To construct a Punnett Square you have to identify the possible gene combinations an organism could produce in the gametes and use those for 'axis' of the Punnett Square. In figure 1 these gamete combinations occupy the dark grey squares at the edge of each table, that is, the 'A' and 'a' (figure 1, square 1) and the 'Ab' (figure 1, square 2) etc., or as can be seen in the figure 2 panel i, below.
Figure 2: Punnett Square 1 - Figure showing a simple 'one allele' situation. The possible gametes are written along the top and the side, and then the square is completed by multiplying the axes. So, 'A' on the top and 'A' on the side gives 'AA' in the square - see panel ii; 'A' on the top and 'Aa' on the side gives 'Aa' - see panel iii
Figure 2 shows a simple 'one allele' situation. The possible gametes are written along the top and the side, and then the square is completed by multiplying the axes. So, 'A' on the top and 'A' on the side give 'AA' in the square - see figure 2 panel ii; 'A' on the top and 'Aa' on the side give 'Aa' - see figure 2 panel iii.
Deciding on the Gametes
You have a male and female Blue Brown Frog, both of which are brown, and both of which carry a recessive gene for blue....
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'
Right, we now have the gametes identified, we can fill in the square...
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