A Punnett square is a diagram used to visualize the possible results of a mating and to predict the genotypic and phenotypic ratios for the offspring it produces. The generation of a Punnett square for a cross between a homozygous tall (TT) and a homozygous short (tt) individual is the subject of the first animation.
In order to construct a Punnett square, a column is created for each type of gamete that the male parent can produce, and a row is generated for each type of gamete that the female parent can produce. Since each parent has two alleles, the result should be a box with two columns and two rows, generating four squares. Next, the alleles for the male parent are placed above the top of each column, while the female parent's alleles are placed to the left hand side of each row. The resulting Punnett square for our TT x tt mating is shown in the second frame of the first animation.
To generate predicted progeny genotypes from this mating, you then simply fill in the boxes with the corresponding male and female alleles. In this example, all of the progeny would get a T from the male parent, and a t from the female parent, and would be tall.
In the second animation, a heterozygous tall parent is crossed with a homozygous short parent. The Punnett square for this mating predicts that half of these progeny will be tall (Tt), while the other half will be short (tt).
It is important, to remember that these are just expected ratios and as such will not necessarily predict the actual outcome of a mating.