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. T is completely dominant for tall pea plants and tt is short. A Tt plant is crossed with a tt plant. What is the expected phenotype ratio?

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

  2. In pea plants, T is completely dominant for tall height and tt is short. Two Tt plants are crossed. What is the expected phenotype ratio?

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

  3. Which term describes Tt, containing two different alleles for one gene?

    • Homozygous
    • Phenotype
    • Heterozygous

  4. T is the dominant allele in a pea-height model. Which term describes the genotype TT?

    • Heterozygous
    • Homozygous dominant
    • Homozygous recessive

  5. T is completely dominant for tall pea plants. A Tt-by-Tt cross predicts 3 tall to 1 short, but four offspring include two tall and two short plants. Why can this happen?

    • The parents changed genotype
    • The allele t became dominant after the first offspring
    • Chance affects the four independent outcomes

  6. T is completely dominant for tall pea plants. The first three offspring from a Tt-by-Tt cross are tall. Under independent fertilization, what is the chance that the next offspring is short?

    • 1/4
    • 1
    • 0
    • 1/2

  7. In a Tt-by-Tt cross with equally likely allele transmission, what fraction of offspring is expected to be homozygous?

    • 1/4
    • 1/2
    • 3/4

  8. In pea plants, T is completely dominant for tall height and tt is short. A TT plant is crossed with a tt plant. What phenotype does this model predict for the offspring?

    • All tall
    • Half tall and half short
    • All short

  9. Complete this Tt-by-Tt Punnett square. Choose the entries in this order: top-left, top-right, bottom-left, bottom-right. Write capital letters first.

    GametetT
    T??
    t??
    • TT, Tt, Tt, tt
    • Tt, TT, tt, Tt
    • Tt, tt, TT, Tt

  10. T is completely dominant for tall pea plants. For 400 offspring from Tt-by-Tt crosses, how many short plants are expected?

    • 100
    • 200
    • 300
LightMySky · lightmysky.comW1-mt_T4ZC45-77w-s1

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. 1 tall to 1 short · The Tt parent supplies T or t with equal chance. The tt parent supplies t. Half the offspring are expected to be Tt and tall; half are tt and short.
  2. 3 tall to 1 short · Tt by Tt gives TT, Tt, Tt and tt. Three equally likely boxes are tall and one is short.
  3. Heterozygous · T and t are different alleles, so Tt is heterozygous.
  4. Homozygous dominant · TT contains two identical copies of the dominant allele, so it is homozygous dominant.
  5. Chance affects the four independent outcomes · Each offspring has a 3/4 chance of being tall and a 1/4 chance of being short. Those probabilities do not require every group of four to contain three tall plants.
  6. 1/4 · A short offspring must be tt. That outcome has probability 1/4 on each independent fertilization, regardless of earlier offspring.
  7. 1/2 · TT and tt are homozygous. Together they occupy two of four equally likely boxes, so the expected fraction is one half.
  8. All tall · The TT parent supplies T and the tt parent supplies t. Every offspring is Tt, which is tall under complete dominance.
  9. Tt, TT, tt, Tt · For the top row, T combines with t and T to give Tt and TT. For the bottom row, t combines with t and T to give tt and Tt.
  10. 100 · Short plants have genotype tt, with probability 1/4. The expected count is 400 x 1/4 = 100. The observed count need not be exactly 100.
Worksheet · LightMySky