Evolution Revised January 2018
www.BioInteractive.org Page 2 of 4
Popped Secret: The Mysterious Origin of Corn
5. Dr. Beadle conducted an experiment to determine how many genes control the differences between maize
and teosinte. He crossed teosinte with maize (the two parental plants) to produce F
1
hybirds, and then
crossed the F
1
plants to produce an F
2
generation (offspring). He then looked at the appearance, or
phenotype of the offspring. Based on classical genetics, he predicted if just one gene was responsible for all
the differences between maize and teosinte, a parental phenotype was expected in one of every four
offspring—in other words ¼ of the offspring would look like maize and ¼ would look like teosinte. If two
genes are involved, one out of every 16 offspring would look like maize and one out of 16 like teosinte. This
relationship can be summarized by this equation: X = (¼)
n
a. In the equation above, X represents the proportion of offspring expected to have a parental phenotype.
What does n represent?
b. Dr. Beadle planted 50,000 plants and discovered that 1 out of 500 offspring had the phenotype of one
parent and 1 out of 500 of the other parent. Approximately how many plants had a teosinte phenotype?
A maize phenotype? What phenotype(s) did the rest of the plants have?
c. Use the equation X = (¼)
n
to explain how Dr. Beadle came to conclude that four or five genes are
responsible for the differences between maize and teosinte.
d. Explain how changes in a small number of genes can result in very different-looking plants.
6. The film describes two independent sources of evidence that have been used to estimate when maize was
first domesticated: genetic evidence and archaeological evidence. Do these two sources of evidence support
each other? Explain your answer.
7. To demonstrate how two different genes can explain the different traits in teosinte and maize, Dr. Doebley
and colleagues used careful breeding to transplant each gene from one type of plant to the other. In the
table below, draw and/or describe the results of each cross and explain what you can infer about the
function of the genes.
