Punnett Squares

The diagram to the right is called a Punnett square, and is used to predict the possible genotypes of offspring from a specific cross. In the scenario below, a corn plant that is hemizygous for the Bt gene is pollinated by itself or by another hemizygous Bt corn plant. There are two possible female gametes (silk/egg), one with the Bt gene, and one without. There are also two possible male gametes (pollen), one with the Bt gene, and one without. When fertilization occurs, the male and female gametes randomly join. There are four possible combinations of chromosome pairs in the offspring.

The next diagram shows that one in four progeny will have 2 copies of the Bt gene, two in four progeny will have one copy of the Bt gene, and one in four will have no copies of the Bt gene and be non-transgenic. If one copy of the Bt gene per cell makes enough of the Bt protein for complete toxicity to the European corn borer (ECB), the Bt gene is considered dominant.

Punnett squares are a 2 x 2 square where the possible female gametes (eggs) and are displayed along the top of a 2 x 2 square and possible male gametes (pollen) are displayed along the left side of the 2 x 2 square. The possible offspring are represented by the each one of the four squares in the 2 x 2 square. Image by P. Hain

Therefore, 3/4 of the plants would be resistant to ECB. If the Bt gene is recessive, only plants with two copies of the Bt gene will express a high enough level of the Bt protein to be toxic. Therefore in this case, only the 1/4 of the plants with two copies would have resistance.

When two hemizygous transgenic plants are mated, (or one hemizygous plant is selfed), the offspring will have a 3:1 ratio of transgenic to non-transgenic plants with 1/4 plants having two copies of the transgene and 1/2 having only one copy. Image by P. Hain

The ratio of possible offspring when a hemizygous plant is selfed or mated with another hemizygous plant is 1/4 homozygous Bt (ECB Resistant), 1/2 hemizygous Bt (ECB resistant), and 1/4 homozygous non-Bt (ECB susceptible). Image by P. Hain

A typical Punnett square used to determine offspring genotypes uses just letters to represent alleles and looks like the example below.

Typically Punnett squares use letters to represent the alleles. Image by P. Hain