A Simulation of the Classic Dihybrid Cross

endosperm: starchy (S) or sweet (s)
aleurone: purple (R) or yellow (r)

The simulated corn cob is generated in the 9:3:3:1 ratio each time the page loads.
Watch it change

This is a simulation of the dihybrid cross RrSs X RrSs, where (R) is the dominant allele for purple aleurone (a cell layer just beneath the seed coat), (r) is the recessive allele for yellow aleurone, (S) is the dominant allele for starchy endosperm (and smooth kernels), and (s) is the recessive allele for sweet endosperm (and wrinkled kernels).

In an ideal cross, we would expect every cell in the Punnett square to contain the same number of progeny (this is more traditionally expressed by combining cells with the same genotype to arrive at the familiar ratio of 1:2:1:2:4:2:1:2:1). We would also expect phenotypes to be expressed in a ratio of 9:3:3:1.

As you know, segregation and recombination are stochastic processes, so the observed ratios often deviate from the expected ones. One page allows you to vary the sample size, so you can observe that larger sample sizes generally give closer ratios. It also calculates chi-squared to show that observed ratios are generally not significantly different from expected ones. Another page allows you to perform multiple runs to better understand probability. You can also download the original MS-DOS program that these pages are based on.
© 2001 by Curtis Clark