Epistasis and Reading Pathway Order from Double Mutants · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

When one gene masks another

Science · Genetics & Evolution · ages 18-19
Name ______________________   Date ____________

  1. Epistasis always involves how many genes?

    • Two
    • One
    • Three

  2. Both dominant alleles are needed for a purple flower. Which ratio do you expect?

    • 3 purple to 1 white
    • 9 purple to 7 white
    • 9 white to 7 purple

  3. Dominance and epistasis describe the same thing.

    Circle one:   True   False

  4. A cross gives 12 of one type, 3 of a second, and 1 of a third. What does this mean?

    • The genes sit close together
    • One allele is lethal
    • One dominant allele hides the other gene

  5. The double mutant looks exactly like the single mutant of gene B. Which gene acts later?

    • Gene A
    • Gene B
    • Both at once

  6. A 9 to 3 to 4 ratio appears in 1600 offspring. How many fall in the last class?

    Answer: ______________

  7. In a pathway, the double mutant always looks like the earlier acting single mutant.

    Circle one:   True   False

  8. A ratio departs from 9 to 3 to 3 to 1. Which test separates linkage from epistasis?

    • Recombination analysis of the genes
    • Counting only the largest class
    • Repeating the parental cross
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Answer key

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

When one gene masks another W1-mt_eE2WRPyYPk-s1

  1. Two · Epistasis is an interaction between two different genes.
  2. 9 purple to 7 white · Needing both dominant alleles gives the 9 to 7 complementary split.
  3. False · Dominance is alleles of one gene; epistasis is two genes interacting.
  4. One dominant allele hides the other gene · 12 to 3 to 1 means one dominant allele masks the second gene.
  5. Gene B · The matching single mutant marks the downstream block.
  6. 400 · The last class is 4 of 16, and 4 times 100 is 400.
  7. False · It matches the later block, since nothing passes a downstream break.
  8. Recombination analysis of the genes · Only linkage answers a recombination analysis; epistasis does not.
Worksheet · LightMySky