From Wild Grass to Golden Grain: A Food Lab Week

Compare how selective breeding and genetic engineering change the traits of food plants.

Part 1: From teosinte to corn

Read the passage and Table 1, then answer questions 1–3.

Foods People Have Changed

1Mr. Kwame Sorensen-Díaz's food science class at Harmon Ridge Middle School spent one week on foods that people have changed. On Monday the class pressed corn tortillas. Then he passed around two ears: a long ear of field corn and a thin ear of teosinte, no longer than a finger, from a seed collection. Teosinte is a wild grass that still grows in Mexico. DNA studies show that corn was bred from teosinte about 9,000 years ago by farmers in what is now southern Mexico.

2For thousands of years, farmers saved seeds from the plants they liked best at each harvest and planted them the next season. Plants of the same kind differ a little in almost every trait. Some teosinte ears had a few more kernels than others, some had thinner kernel cases, and some held together a little longer when ripe. Table 1 compares teosinte with corn today.

3On Wednesday, Bettina Aroyo's group tested apples. When an apple is cut, an enzyme from its cells mixes with oxygen from the air and turns the cut surface brown. The group cut slices from a regular apple and from a non-browning apple of the same variety. To make the non-browning apple, scientists added a piece of DNA to the apple tree's cells that switches down the tree's own genes for the browning enzyme. New trees grafted from the engineered tree also grow non-browning apples. Mr. Sorensen-Díaz cut all of the slices at the same time so that every group started together. The group also dipped some regular slices in lemon juice, a common kitchen trick. All of the slices sat on plates at room temperature, and the group scored browning from 0 (none) to 5 (fully brown). Table 2 shows the average score of six slices in each group.

4On Friday the class read about golden rice. Rice plants make beta-carotene, an orange substance the body turns into vitamin A, in their green leaves but not in their grain, and plant breeders have not found any rice plants that store it in the grain. So scientists added two genes to rice, one from corn and one from a soil bacterium. The grains of golden rice are yellow-orange because they hold beta-carotene. In places where people eat mostly rice, too little vitamin A can harm children's eyesight.

Table 1: Teosinte and Corn Today
Trait	Teosinte	Modern field corn
Ears per plant	Many small ears on side branches	1 or 2 large ears
Kernels per ear	About 10	About 600
Kernel cover	Hard, stony case	Soft, kernels exposed
Ripe ear	Breaks apart, seeds fall	Stays whole on the cob

Use Table 1.

An ear of modern field corn has about times as many kernels as an ear of teosinte.

Teosinte ears break apart when ripe, but corn ears stay whole. Which explanation of this change fits paragraphs 1 and 2 best?

A. Over many harvests, farmers planted seeds from ears that held together longer.
B. Scientists added a gene for whole ears to teosinte plants about 9,000 years ago.
C. Each teosinte plant learned to hold its ears together once farmers began to pick them.
D. Ears that broke apart spread more seeds, so wild teosinte slowly turned into corn.

Corn seldom keeps growing in a field for long once people stop planting it. Which trait in Table 1 best explains this?

A. About 600 kernels on each ear
B. One or two large ears on each plant
C. Ripe ears that stay whole on the cob

Part 2: Apples and golden rice

Use Table 2 and paragraphs 3 and 4 to answer questions 4–9.

Table 2: Average Browning Score (0 = none, 5 = fully brown)
Time after cutting	Regular slices	Regular + lemon juice	Non-browning slices
0 h	0	0	0
1 h	2	0.5	0
4 h	3.5	1	0.5
24 h	5	2.5	1

Use Table 2.

At 4 h, the regular slices scored points higher than the non-browning slices. At 24 h, the lemon-juice slices scored points higher than the non-browning slices.

Which conclusion do the results in Table 2 support best?

A. The non-browning slices browned least at each check after cutting.
B. Lemon juice kept slices as pale as the non-browning apple did at 24 h.
C. The dipped and regular slices had the same score at the 24 h check.
D. The regular slices stopped browning after the first hour on the plate.

Bettina says, "Lemon juice and the genetic change do the same thing." Which difference between them matters most?

A. Both change the apple's DNA, but the genetic change wears off after about a day.
B. The genetic change is in the tree's DNA, so grafted trees keep it; juice affects one slice.
C. Lemon juice changes the apple's enzyme genes, so later apples from that tree brown less.
D. The non-browning slices turned brown faster than the dipped slices in the first hour.

Use the whole passage. Match each food to the way people produced its trait.

Corn ears that stay whole when ripe
Golden rice grains that hold beta-carotene
Non-browning apples from grafted trees
Dipped regular slices that brown more slowly

A. Coating the cut surface, with no change to genes
B. Saving seeds from chosen plants for many generations
C. Adding DNA that switches down the plant's own genes
D. Adding genes from two other species

Use paragraphs 2 and 4. Why did scientists use genetic engineering, not artificial selection, to make golden rice?

A. Rice plants make no beta-carotene in any part, so a corn gene was needed.
B. Breeders can select a trait only if some plants have it, and no rice grain did.
C. They wanted larger grains, and the corn gene makes each rice grain bigger.
D. Artificial selection is a method for farm animals, not for grain crops.

A classmate says, "Corn and the non-browning apple were made the same way. People just changed the plants they grew." Write a claim that answers the classmate. Use evidence from Table 1, Table 2 and the passage, and explain how each technology changed the plant's traits.