Punnett Squares and Monohybrid Ratios · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Predicting offspring with a grid

Science · Genetics & Evolution · ages 15-16
Name ______________________   Date ____________

  1. Two heterozygous tall pea plants (Tt) are crossed. What is the expected phenotype ratio?

    • 3 tall to 1 short
    • 1 tall to 3 short
    • 2 tall to 2 short
    • 4 tall to 0 short

  2. Two heterozygous tall plants (Tt) are crossed. What is the expected phenotype ratio?

    • 3 tall to 1 short
    • 1 tall to 3 short
    • 4 tall to 0 short

  3. Tt has two different alleles. What term describes it?

    • Homozygous recessive
    • Pure breeding
    • Heterozygous

  4. TT has two identical alleles. What term describes it?

    • Heterozygous
    • Homozygous dominant
    • Recessive phenotype

  5. A Tt by Tt cross predicts 3 to 1, but a litter of four has two of each. Why?

    • The grid fails for small families
    • Each birth is independent, and small samples vary
    • Recessive alleles vanished this time

  6. A Tt x Tt cross predicts 3 tall to 1 short, but a litter of four has two of each. Why?

    • Ratios are probabilities, and small samples vary widely
    • The Punnett square only works for animals
    • Dominant alleles always disappear
    • Heterozygous parents cannot have short offspring

  7. In a Tt by Tt cross, what fraction of offspring are homozygous?

    • One quarter
    • One half
    • Three quarters

  8. A TT plant is crossed with a tt plant. What will the offspring look like?

    • All tall, since each is Tt
    • Half tall and half short
    • All short, since t doubles up

  9. A student fills a Tt by Tt grid as TT, TT, tt, tt and claims 1 to 1. What is the error?

    • The grid edges were mislabeled
    • The boxes were miscounted, since two boxes must be Tt
    • Ratios never apply to pea plants

  10. In 400 offspring of Tt by Tt parents, about how many short plants do you expect, and why?

    • About 100, since one box in four is tt
    • About 200, since parents split evenly
    • About 300, since tall dominates
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Answer key

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

Predicting offspring with a grid W1-mt_T4ZC45-77w-s1

  1. 3 tall to 1 short · Tt x Tt gives TT, Tt, Tt, tt: three tall to one short.
  2. 3 tall to 1 short · The boxes read TT, Tt, Tt, tt, so three are tall and one is short.
  3. Heterozygous · Mixed pairs are heterozygous, with the dominant trait showing.
  4. Homozygous dominant · Identical pairs are homozygous, and T is the dominant allele.
  5. Each birth is independent, and small samples vary · Probabilities settle near predictions only in large numbers.
  6. Ratios are probabilities, and small samples vary widely · A 3 to 1 ratio is a probability per birth, so litters of four rarely match it exactly.
  7. One half · TT and tt are the two homozygous boxes out of four.
  8. All tall, since each is Tt · Every box gets T from one parent, so every plant is Tt and tall.
  9. The boxes were miscounted, since two boxes must be Tt · Correct filling gives TT, Tt, Tt, tt, which reads 3 to 1.
  10. About 100, since one box in four is tt · One quarter of 400 is 100, and big samples land near the ratio.
Worksheet · LightMySky