More Corn Genetics

Corn is not only a commercially important crop, it has also been a valuable organism used in basic genetic studies to understand genetic principles. Geneticists such as Dr. John Osterman, in the School of Biological Sciences at the University of Nebraska, use corn in basic genetic studies. Let’s examine one experiment.

Many seed traits in corn have been studied by geneticists. Color in the outside (alluerone) or inside (endosperm) of the seed can be important for end uses such as what color of corn chips you prefer. Starch development traits in the kernel influence what the corn seed can be used for (popcorn, sweet corn, waxy corn).

A line of corn that was true breeding for a red kernel color was crossed to a true breeding line with shrunken kernels but no red color (white or yellow). Even though the two parents differed by two traits, kernel color and kernel shape, the F1 offspring produced from the cross were all red with plump or normal kernels.

Parents:     Red, plump     X       white, shrunken
F1 Offspring:     All red, plump    

Which phenotypes would you say are dominant based upon this single result? Going a step further, what is your hypothesis for the genotypes of the parents and F1?

The next year these red plump seeds were planted in the crossing nursery near the white shrunken line. Dr. Osterman made a backcross or testcross between the F1 progeny and white, shrunken plants.

Thinking back on the genetics principles we have covered so far in this course, what phenotypes would you expect to observe in the testcross progeny?

If we use the principle of independent assortment and the information we already have, we would predict that there should be four types of seeds in testcross progeny.

Fig 1. Seed trait inheritance in test cross progeny assuming independent assortment. (Image credit: D. Lee)

We should get both parental combinations of red, plump and white, shrunken We should also get two new combinations; red, shrunken and white, plump Furthermore, the principle of independent assortment predicts the four combinations should be in a 1:1:1:1 ratio. (Make sure you understand why!)

What did Dr. Osterman actually observe? When he harvested the ear from the first plant, peeled back the husks, and examined the kernels, all he saw were red, plump and white, shrunken seeds. Only the parental combination of traits was found among the several hundred seeds on that ear!

Fig. 2. Testcross progeny showing red, plump and white, shrunken phenotype. (Image credit: D. Lee)

Half the seeds had the dominant red and plump combination while the other half had the recessive traits. While this result does not fit what we expect based on the idea of independent traits, Dr. Osterman was not surprised. Obviously another genetic hypothesis can explain this result. Let’s examine two alternative explanations.