Behind the Team's Training Table: Bred and Engineered Foods

Use chicken growth data and a papaya field test to explain how people change traits.

Answer Key

Part 1: Bred for growth

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

What's on the Plate

1The cross-country team at Pinecrest Middle School eats a team dinner before every big race: grilled chicken, a slice of cheese and a papaya smoothie. Eighth grader Bashir Calloway, one of the team's runners, offered to give a short talk at the next dinner about where these foods come from. His science teacher, Ms. Ingrid Solberg, suggested that he find out how people have changed the traits of each one.

2Bashir started with the chicken. For about 70 years, poultry breeders have chosen the fastest-growing birds in each flock to be the parents of the next generation. A chicken can become a parent when it is about half a year old, so breeders have gone through many generations since the 1950s. University scientists kept small flocks of a meat-chicken strain from 1957 and a strain from 1978 that were not bred any further. They compared them with a strain from 2005. All three strains were raised in the same barn, ate the same feed and were weighed at 56 days old. Table 1 shows the results.

3Some of Bashir's teammates guessed that the birds are fed hormones. In the United States, federal rules do not allow hormones to be given to chickens. Bashir also read that some very fast-growing birds have more leg and heart problems, so many breeders now choose parents for health and strong legs as well as for growth.

Table 1: Average Mass of Meat Chickens at 56 Days
Strain from year	Average mass (kg)
1957	0.9
1978	1.8
2005	4.2

Use Table 1. Round to the nearest tenth.

At 56 days, the 2005 strain was about how many times as heavy as the 1957 strain? 4.7 times

Use Table 1.

How much more mass did the 2005 strain have at 56 days than the 1978 strain? 2.4 kg

Why was it important that the scientists raised all three strains in the same barn on the same feed?

A. So a difference in mass would come from inherited traits
B. So the 1957 birds would grow as fast as the newer birds
C. So the birds would eat less feed than they did on farms
D. So the birds could pass their growth to the other strains

Which description best explains how breeders produced the change shown in Table 1?

A. They fed each bird more each year, and the birds passed the extra mass to their chicks.
B. They chose fast growers as parents again and again, so fast-growth genes became common.
C. They gave the chicks hormones, and each new strain kept the hormones in its body.
D. They moved a growth gene from another species into the chickens about 50 years ago.

What does paragraph 3 suggest about a risk of artificial selection?

A. Choosing parents for strong legs makes the birds grow more slowly.
B. Choosing parents mainly for one trait can bring other problems along.
C. Choosing parents for growth stops the birds from having chicks.
D. Choosing parents for growth makes the chicks need hormones to survive.

Part 2: Moving genes

Read the passage and Table 2, then answer the questions.

Cheese and Papaya

1Next, Bashir looked at the cheese. Making most cheese needs an enzyme called chymosin, which makes milk thicken into curds. For a long time, chymosin came mainly from the stomachs of young calves. Around 1990, scientists copied the calf gene for chymosin and put it into microbes, such as certain yeasts and molds. The microbes are grown in large tanks, and they make the same enzyme the calf gene codes for. Today most cheese made in the United States uses chymosin made this way.

2Then he looked at the papaya. In the 1990s, a virus called papaya ringspot, carried from tree to tree by small insects called aphids, damaged many papaya farms in Hawaii. Scientists put a gene from the virus itself into papaya plants, and the engineered plants could fight off the virus. In one test field, 60 engineered trees and 60 regular trees were planted side by side at the same time. Two years later, workers checked every tree for signs of the virus. Table 2 shows what they found.

Table 2: Papaya Test Field After Two Years
Kind of tree	Trees planted	Trees with the virus
Engineered	60	3
Regular	60	54

Use Table 2.

What percent of the regular trees had the virus? 90%
What percent of the engineered trees had the virus? 5%

Which conclusion does the test field best support?

A. The added gene made the engineered trees far less likely to get sick.
B. The regular trees got sick because they were planted two years later.
C. The engineered trees passed their resistance to the regular trees.
D. Aphids stayed away from the test field, so few trees became sick.

Match each food to how people changed it.

Meat chickenA
Chymosin for cheeseC
Ringspot-resistant papayaB

A. Breeders chose parents with a trait for many generations.
B. A gene from a virus was put into the plant.
C. A calf gene was put into microbes grown in tanks.

What is one advantage of making chymosin with microbes, based on paragraph 1?

A. The microbes turn milk into cheese by themselves inside the tanks.
B. Cheese made with it comes from cows that were genetically engineered.
C. The microbes make a new enzyme that curdles milk better than chymosin.
D. Large amounts of the same enzyme can be made without calf stomachs.

10.

Which statement best compares the two technologies on this page?

A. Engineering left the papaya's genes alone; selection changed the chickens' genes.
B. Selection moved genes between species; engineering used traits already there.
C. Engineering added a gene in one step; selection worked over many generations.
D. Selection changed the chickens in one generation; engineering took decades.

11.

A teammate says, "Chickens today are bigger only because farmers feed them better than they did in 1957." Make a claim about what caused the change in chicken size. Use data from Table 1 and details about how the study was done as evidence, and explain your reasoning.

Claim: The teammate is wrong; chickens are bigger mainly because artificial selection changed their inherited traits. Evidence: In the study, all three strains ate the same feed in the same barn, yet at 56 days the 1957 strain averaged 0.9 kg, the 1978 strain 1.8 kg and the 2005 strain 4.2 kg, about 4.7 times as heavy. Hormones are not allowed in US chickens. Reasoning: Because the food and care were the same, better feeding cannot explain the difference. The strains differ in their genes, because breeders chose the fastest-growing birds as parents for many generations, so genes for fast growth became common.