Two tomato plants grown from cuttings of the same parent plant share one greenhouse. The fertilized plant reaches 90 cm, and the unfertilized one reaches 40 cm. What best explains the height difference?

Six puppies from one litter are raised together on the same food and the same exercise. One grows noticeably taller than its littermates. What best explains that difference?

A farmer sows a short-stemmed wheat variety in a dry field and a tall-stemmed variety in a well-watered field. The tall variety ends up far taller. What explains that height gap?

Goldfish hatched from the same parents are split between a small aquarium and a large garden pond. A year later the pond fish are much longer. What best accounts for the size difference?

Two cuttings taken from one hydrangea shrub are planted in different beds. One blooms blue and the other pink, although both received the same water and light. What best explains the color difference?

Two brothers inherit genes for tall height. One is well fed throughout childhood and reaches 190 cm, and the other, poorly fed for years, reaches 172 cm. What determines adult height here?

A gardener sows seeds of a dwarf pea variety and a tall pea variety in identical pots with the same soil, water and light. The tall variety grows twice as high. Why?

A student tests whether a fertilizer makes cabbages heavier. She fertilizes plants of one cabbage variety and leaves plants of a different variety unfertilized. What change would let her answer her question?

Two chicken breeds are compared to find out whether one is genetically heavier. One breed was raised on high-protein feed and the other on ordinary feed. What must change for the comparison to work?

A team grows cuttings taken from a single willow tree at three light levels, keeping soil and water the same for all of them. Their trunk-growth measurements can answer which question?

Two plots of the same corn hybrid receive different amounts of nitrogen, and their yields differ. A student concludes that the corn in the two plots differs genetically. What is wrong with that conclusion?

To test whether a height difference between two bean varieties is inherited, a student grows a single seed of each variety. Besides matching their growing conditions, what else should she change?

Clones of one strawberry plant are grown in full sun and in deep shade. The sun-grown group puts out far more runners. What claim does this evidence support?

Identical twin sisters live in one house and eat the same meals. A scientist studying how diet affects body mass says this pair is a poor test of her question. Why is she right?

Identical twin sisters carry the same genes. One trains as a distance runner and can run far longer without tiring than the other, who rarely exercises. What does this comparison show?

Any young female honeybee larva in a hive can develop into a worker or a queen. The few fed royal jelly right through development become large, fertile queens. What explains the difference?

Cells from one bacterial colony are spread on two dishes. The dish holding rich nutrient jelly grows a thick film of bacteria; the dish holding poor jelly grows a thin one. What does this show?

A potato tuber is cut into pieces and each piece is planted. Pieces in deep, moist soil grow large plants; a piece in a dry, stony corner grows a small one. What does the comparison show?

Identical twin brothers carry the same genes. One works outdoors on a farm and has darker, more weathered skin than the other, who works indoors all day. What does this comparison show?

Cuttings taken from one coleus plant are grown in bright sun and in deep shade. The shaded plants develop broader, thinner leaves. What best explains the leaf difference?