Dandelion Seeds and Checkered Pigeons

Compare one-parent and two-parent offspring and use a Punnett square.

Answer Key

Part 1: Seeds from the weedy lot

Read the passage and Tables 1 and 2, then answer the questions.

A Year at Brookfield Commons

1Emeka Haugland's sixth-grade science club spends a year studying Brookfield Commons, a city park with lawns, a weedy lot beside the parking area and a community center. The park, the people and the numbers here are made up, but they are based on real studies.

2In spring and early summer, the club studied two common weeds from the lot: dandelions and chicory. Both have flower heads made of many tiny flowers, and each tiny flower can make one seed. In many plants, a seed forms only after pollen reaches the flower and a sperm cell joins an egg cell, so the seed gets genetic information from two parents. Chicory works this way, and its flowers usually need pollen from a different chicory plant. To test the plants, the club's adviser, Ms. Henrietta Vos, used small scissors to slice the top off closed flower heads. Slicing removes the parts that make and catch pollen but leaves the bottom of the head, where seeds grow. After the tiny flowers had closed, the club tied a mesh bag over each head to catch its seeds.

3In June, the club planted 24 seeds from a sliced head of one dandelion plant, D1, and 24 seeds from one chicory plant, C1, whose flowers had been visited by bees carrying pollen from other chicory plants. All 48 seeds went into the same garden bed by the community center, with the same soil, sun and watering. Ten weeks later, the club compared the young plants.

4A pigeon club keeps a loft beside the community center, and in the fall its members showed Emeka their breeding records. Pigeon pairs usually raise two young, called squabs, at a time, several times a year. In these pigeons, wing pattern is controlled by one gene with two alleles. The checker allele (C) gives a checkered pattern and is dominant over the bar allele (c), which gives two dark bars across each wing. Each parent passes one of its two alleles, chosen at random, to each squab.

Table 1. Flower Head Test (20 heads in each group)
Plant	Flower heads	Heads that made seeds	Seeds per head (avg.)
Dandelion	Left whole	20	180
Dandelion	Top sliced off	19	170
Chicory	Left whole	18	14
Chicory	Top sliced off	0	0

Use Table 1.

Of the dandelion heads with the top sliced off, 95% still made seeds.

Why did the club slice chicory heads as well as dandelion heads?

A. To check whether slicing stops seeds in a plant that needs pollen
B. To find out which plant's flowers open first in the morning
C. To make the chicory bloom at the same time as the dandelions
D. To collect extra chicory seeds for the June planting

Which conclusion do the four rows of Table 1 support best?

A. Sliced dandelions made 10 fewer seeds, so most dandelion seeds need pollen.
B. Chicory needs no pollen to make seeds, while dandelions need a lot.
C. These dandelion seeds formed without pollen, so they came from one parent.
D. Slicing the top off a flower head makes it form seeds faster.

Table 2. Young Plants After 10 Weeks (24 from each parent)
Seeds from	Leaf shapes seen	Longest leaf, range (cm)
Dandelion D1	1	14 to 17
Chicory C1	4	9 to 21

"Longest leaf, range" gives the shortest and the longest of the 24 plants' longest leaves.

Use Table 2. Subtract to find each spread.

The longest leaves of the D1 plants differed by up to 3 cm. The longest leaves of the C1 plants differed by up to 12 cm.

All 24 D1 plants had the same leaf shape, yet their longest leaves still differed by a few centimeters. Which explanation fits the passage best?

A. Each D1 seed got a different mix of genes from two parent plants.
B. The seedlings' genes changed as they grew, so each one became different.
C. The seeds came from many different dandelion plants in the lot.
D. Small differences in each plant's spot, like light or water, affected growth.

Use the passage and Tables 1 and 2. Sort each statement by the young plants it describes.

Got genetic information from two parent plantsHave the same genetic information as their one parentGrew from seedsShowed four different leaf shapesCame from a kind of plant whose sliced heads still made seedsGrew in the same bed with the same soil and sun

D1 dandelion plants	C1 chicory plants	Both groups
Have the same genetic information as their one parent Came from a kind of plant whose sliced heads still made seeds	Got genetic information from two parent plants Showed four different leaf shapes	Grew from seeds Grew in the same bed with the same soil and sun

Part 2: Squabs in the pigeon loft

Use paragraph 4 and Table 3.

Table 3. Squabs Raised This Year
Pair	Parents' wings	Checker squabs	Bar squabs
1	Checker × checker	6	2
2	Checker × bar	4	4
3	Bar × bar	0	8
4	Checker × bar	8	0

Use paragraph 4.

Using C and c, the genotype of a pigeon with bar wings is cc.

Both Pair 1 parents have checker wings, but they raised bar squabs. Use the Punnett square for Pair 1.

Punnett Square: Pair 1
Parent 1 ↓ / Parent 2 →	C	c
C	CC	?
c	?	?

Each Pair 1 parent's genotype is Cc. The box where c meets c holds cc. For each squab of Pair 1, the chance of bar wings is 25%.

Pair 1 raised 8 squabs this year.

The Punnett square predicts about 2 squabs with bar wings out of 8.

10.

Pairs 2 and 4 are both checker × bar. Why did Pair 2 raise bar squabs while Pair 4 raised none?

A. Pair 2's checker parent is Cc, while Pair 4's checker parent is most likely CC.
B. Pair 4's checker parent passed both of its alleles to each of its squabs.
C. Pair 2's squabs hatched in summer, when warm weather gives squabs bar wings.
D. Pair 4's bar parent carries a hidden checker allele that it gave to its squabs.

11.

Next year, dandelion D1 makes 8 more seeds the way its sliced heads did, and Pair 1 raises 8 more squabs. Which statement compares the offspring best?

A. D1's seeds would vary more, because a plant makes more offspring than a pigeon pair.
B. D1's seeds would share its genetic information, while Pair 1's squabs could be checker or bar.
C. Pair 1's squabs would have checker wings, the same as both of their parents.
D. Both sets of offspring would be exact genetic copies of a single parent.

12.

Over the year, Emeka saw dandelions spread quickly across a newly seeded lawn, while the loft's squabs often looked different from their brothers and sisters. Which kind of reproduction gave more genetic variation in the park? Write a claim, give evidence from Table 2 and Table 3, and use a model of how genetic information passes from parents to offspring to explain your reasoning.

Claim: sexual reproduction, as in chicory and the pigeons, gave more genetic variation than the dandelions' asexual seeds. Evidence: the 24 D1 dandelion plants all had one leaf shape and their longest leaves differed by only 3 cm, while the 24 C1 chicory plants had four leaf shapes and differed by 12 cm, even in the same bed. In Table 3, Pair 1 (checker × checker) raised 6 checker and 2 bar squabs, so brothers and sisters differed. Reasoning: a dandelion seed made without pollen gets all of its genetic information from one parent, so the offspring are copies and differ only a little because of their environment. In sexual reproduction, each parent passes one of its two alleles at random to each offspring, so different offspring get different combinations. For Pair 1 (Cc × Cc), the Punnett square gives CC, Cc or cc, and each squab has a 25% chance of being cc with bar wings. Asexual reproduction is fast, which may help dandelions spread across a new lawn, but sexual reproduction makes offspring that are more varied.