Seed Potatoes and Hornless Calves

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Seed Potatoes and Hornless Calves

Compare clones and crosses on a family farm, then use Punnett squares to predict calves.

The potato trial

Read the passage and Table 1. Then answer questions 1–5.

Spring and Summer at Two Creeks Farm

1Bodhi Larsen is in sixth grade, and his family runs Two Creeks Farm, where they grow potatoes and raise beef cattle. The farm, the people and the numbers here are made up, but they are based on real farm practices. This year Bodhi kept the records for two farm projects as a science project.

2Like most potato farmers, the Larsens plant "seed potatoes." These are not seeds at all. They are pieces cut from potatoes, the swollen underground stems called tubers. Each piece has a bud, or eye, that grows into a new plant. Potato plants also make flowers, and a flower that gets pollen can form a small green berry full of tiny true seeds. The berries are poisonous, so they are never eaten.

3Last year, Bodhi's mother dusted pollen from a yellow-skinned variety called Gold Bell onto flowers of the red-skinned variety they always grow, Red Ridge. This spring, Bodhi planted two plots side by side in the same field. Plot 1 got 60 tuber pieces cut from Red Ridge potatoes. Plot 2 got 60 seedlings grown from the true seeds in those hand-pollinated Red Ridge berries. In July, a week of cool, wet weather spread potato blight, a disease that browns and kills the leaves, across the field.

4The Larsens' cattle come in two forms: some have horns and some are polled, which means hornless. The trait is controlled by one gene. The allele for polled (P) is dominant, and the allele for horns (p) is recessive. The farm's bull, Dakota, is polled, but his mother had horns.

5Horns can injure other cattle, so many farmers have a calf's horn buds removed when it is a few weeks old. Rosie, one of the Larsens' cows, was born with horn buds that were removed when she was a calf, so she has never had horns.

Table 1. Bodhi's Potato Plots (60 plants each)
Plot	Planted from	Skin colors at harvest	Plants with blight	Harvest per plant (kg)
1	Red Ridge tuber pieces	All red	54	1.2
2	Seeds from crossed berries	Red, pink or yellow	21	0.5

1.

What percent of the plants in Plot 1 got blight? %

2.

Every plant in Plot 1 made red-skinned potatoes like Red Ridge. Which explanation fits paragraphs 2 and 3 best?

A. The plants got half their genes from Red Ridge and half from Gold Bell.B. The plants grew from pieces of one variety, so they had its genes.C. The plants were pollinated by bees from Plot 2, which carried red pollen.D. The plants grew in red soil, and the soil colored the skins of their potatoes.

3.

Both plots had the same week of wet weather in the same field. Why did far fewer plants in Plot 2 get blight?

A. The seedlings got used to the blight during the wet week and changed their genes.B. The seedlings varied in their genes, so some likely had alleles that helped them resist.C. The seedlings were shaded by the taller Plot 1 plants, so their leaves stayed dry.D. The seedlings were too young to catch blight, since it attacks plants in their second year.

4.

Which trade-off of planting tuber pieces does Table 1 show best?

A. Tuber pieces gave mixed colors, but the plants resisted blight better.B. Tuber pieces gave a smaller harvest, but each plant had a different skin color.C. Tuber pieces gave a bigger, more even crop, but nearly all got the same disease.D. Tuber pieces gave a smaller harvest, and the plants also got more blight.

5.

Sort each example from the farm by the kind of reproduction that made it.

Horns or no horns

Use paragraphs 4 and 5, the Punnett square and Table 2 to answer questions 6–12.

Table 2. Dakota's Calves, Last Three Years
Cows bred to Dakota	Cows' genotype	Calves	Polled calves	Horned calves
Horned cows	pp	24	13	11
Polled, with horned mothers	Pp	16	12	4

6.

Use paragraph 4. Dakota is polled, and his mother had horns.

Using P and p, Dakota's genotype is .

7.

This Punnett square shows Dakota crossed with a horned cow. Two boxes are missing.

Punnett Square: Dakota × Horned Cow
Dakota ↓ / Cow →	p	p
P	Pp	Pp
p	?	?

The two missing boxes are both . For each calf of this cross, the chance of horns is %.

8.

Use your Punnett square and the first row of Table 2.

Of the 24 calves from horned cows, the Punnett square predicts about horned calves.

9.

Bodhi counted 11 horned calves from the horned cows, not the number the Punnett square predicts. What is the best conclusion?

A. Horns are dominant after all, since the farm got so many horned calves.B. The calves inherited their horns from the field, since few calves were born indoors.C. Dakota is likely PP, since a Pp bull would have sired more horned calves than that.D. The result is close to the prediction; chance causes small differences like this.

10.

Use paragraph 5. Bodhi's uncle says, "Rosie has never had horns, so her calves with Dakota will be polled." Which response fits the passage best?

A. He is wrong, because Rosie still has pp alleles, so about half her calves will likely have horns.B. He is right, because removing her horn buds turned her pp alleles into Pp alleles.C. He is right, because a cow passes on the body she has now, so Rosie passes on no horns.D. He is wrong, because calves get their horn alleles from the bull and not from the cow.

11.

The Larsens want to stop removing horn buds. They plan to replace Dakota with a bull whose calves will be polled with any cow in the herd. Which bull should they choose?

A. A PP bull, because every calf gets one dominant P allele from him.B. A pp bull, because both of his alleles would match the horned cows.C. A Pp bull whose own horn buds were removed when he was a calf.D. A Pp bull, because half of his alleles are the dominant P allele.

12.

Bodhi's mother wants a new potato variety that resists blight, and then a whole field of plants just like it. Which kind of reproduction should she use to find a resistant plant, and which kind should she use to fill the field with copies of it? Write a claim, give evidence from Table 1 and the passage, and explain your reasoning about genetic variation.