Morphogen Gradients and Positional Information · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

How gradients paint sharp stripes

Science · Organisms & Life Processes · ages 20-21
Name ______________________   Date ____________

  1. How does a smooth gradient make a sharp stripe boundary?

    • Cells migrate to sorted piles
    • The source blinks on and off
    • Fates flip where concentration crosses a threshold

  2. What do concentration thresholds produce from a graded signal?

    • Identical fates everywhere
    • Discrete fates from a graded signal
    • Random fates with no pattern

  3. A morphogen always builds the same structure wherever it acts.

    Circle one:   True   False

  4. The morphogen source is removed. What happens to the dependent fates?

    • They appear earlier than ever
    • Dependent fates never appear
    • They swap positions neatly

  5. The morphogen source is doubled. What happens to the pattern?

    • Boundaries slide outward; high-fate zones widen
    • All boundaries freeze in place
    • The pattern flips upside down

  6. Two thresholds divide a gradient. Type how many fate zones result.

    Answer: ______________

  7. Why does one morphogen sculpt different organs in two tissues?

    • Each tissue offers different ready genes
    • The molecule changes its shape
    • Thresholds stop working there

  8. A student says cells count exact molecule numbers. What is wrong?

    • Molecules cannot be counted at all
    • Gradients contain no molecules
    • Cells only compare concentration against thresholds
LightMySky · lightmysky.comW1-mt_EbOK5M7V6Y-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

How gradients paint sharp stripes W1-mt_EbOK5M7V6Y-s1

  1. Fates flip where concentration crosses a threshold · The flip line sits exactly at the crossing point, so the edge is crisp.
  2. Discrete fates from a graded signal · Each threshold flips gene expression, turning a slope into distinct fates.
  3. False · Each tissue offers different ready genes, so one signal sculpts different structures.
  4. Dependent fates never appear · With no signal, no cell crosses any threshold, so the fates vanish.
  5. Boundaries slide outward; high-fate zones widen · Higher dose lifts every level, pushing each threshold crossing outward.
  6. 3 · Two cut lines carve three zones: high, middle, and low.
  7. Each tissue offers different ready genes · The same concentration triggers different gene sets in different programs.
  8. Cells only compare concentration against thresholds · Reading is relative to thresholds, not an exact census of molecules.
Worksheet · LightMySky