Testing Genetic Ratios with Chi-squared · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Testing ratios with chi-squared

Science · Genetics & Evolution · ages 17-18
Name ______________________   Date ____________

  1. How many degrees of freedom test a two class 3:1 ratio?

    • Three
    • One
    • Sixteen

  2. When do geneticists reach for the chi-squared goodness of fit test?

    • To test one cross of counts against a predicted ratio
    • To prove a predicted ratio perfectly true
    • To count chromosomes under a microscope

  3. A small chi-squared value proves the predicted ratio is exactly true.

    Circle one:   True   False

  4. How many degrees of freedom test a four class 9:3:3:1 ratio?

    • Four
    • Sixteen
    • Three

  5. A cross of 320 offspring should follow a 3:1 ratio. How many recessive offspring are expected?

    Answer: ______________

  6. How is the chi-squared statistic built from the counts?

    • Square each gap, divide by expected, then add up
    • Add all gaps, then square the grand total
    • Multiply observed by expected in each class

  7. A cross gives a huge statistic with a tiny p value. What is the verdict?

    • Accept the predicted ratio as proven
    • Collect more offspring before deciding anything
    • Reject the predicted ratio at this sample size

  8. One class shows 95 observed against 100 expected. What does it contribute to chi-squared?

    Answer: ______________

LightMySky · lightmysky.comW1-mt_YCO6jijrNQ-s1

Answer key

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

Testing ratios with chi-squared W1-mt_YCO6jijrNQ-s1

  1. One · Degrees of freedom equal categories minus one, so two classes give one.
  2. To test one cross of counts against a predicted ratio · One batch of offspring against one predicted distribution is its home ground.
  3. False · A pass only keeps the ratio plausible. The test never proves anything true.
  4. Three · Four categories minus one gives three degrees of freedom.
  5. 80 · Recessives are one quarter of a 3:1 ratio, and one quarter of 320 is 80.
  6. Square each gap, divide by expected, then add up · Each class contributes its scaled squared gap, and the sum is the statistic.
  7. Reject the predicted ratio at this sample size · Chance would rarely deal such a gap, so the ratio is rejected.
  8. 0.25 · The gap is 5, squared is 25, and 25 divided by 100 is 0.25.
Worksheet · LightMySky