TEAS Science · Practice Question

    Mendelian Genetics Concepts: TEAS Practice Question with Answer & Explanation

    An original TEAS 7-style science practice question, written for nursing-school applicants — with a full breakdown of every answer choice.

    Question

    In a species of flowering plant, tall stems (T) are completely dominant over short stems (t). A researcher crosses a tall plant with a short plant, and the resulting offspring consist of 24 tall plants and 26 short plants. Which of the following represents the most likely genotypes of the parent plants?

    • A.TT × tt
    • B.Tt × Tt
    • C.Tt × tt
      Correct
    • D.TT × Tt

    Correct answer: C. A 1:1 trait ratio in offspring indicates a cross between a heterozygous parent (Tt) and a homozygous recessive parent (tt).

    Understanding the Punnett Square Results

    To solve this Mendelian genetics problem, we first need to look at the ratio of the offspring. The researcher observed 24 tall plants and 26 short plants. This is approximately a 1:1 ratio (or 50% tall and 50% short).

    Step-by-Step Analysis

    1. Identify the Phenotypes of the Parents: The problem states one parent is tall and the other is short.
    2. Determine the Genotype of the Short Parent: Since short stems (t) are recessive, the only way a plant can physically appear short is if it has two recessive alleles. Therefore, the short parent must be tt.
    3. Determine the Genotype of the Tall Parent: A tall plant could be either homozygous dominant (TT) or heterozygous (Tt).
      • If the parent were TT, all offspring would receive a 'T' and would all be tall.
      • Since we have short offspring, the tall parent must carry the recessive 't' gene to pass it on. This makes the tall parent Tt.

    Evaluating the Choices

    • Choice A: TT × tt — This cross would result in 100% tall offspring (all Tt). This does not match our 1:1 data.
    • Choice B: Tt × Tt — This cross (heterozygous x heterozygous) results in a 3:1 ratio (75% tall, 25% short). This does not match our 50/50 data.
    • Choice C: Tt × tt — This is a test cross. The Punnett square yields two Tt (tall) and two tt (short) squares. This matches the 50% tall and 50% short result observed by the researcher.
    • Choice D: TT × Tt — This cross would result in 100% tall offspring (50% TT, 50% Tt).

    Why This Matters for the TEAS

    Genetics questions on the TEAS often require you to work backward from the offspring to the parents. Understanding the Test Cross (crossing an unknown dominant phenotype with a homozygous recessive individual) is a frequent topic in the Life Science section.

    Memory Tip

    If you see a 50/50 split in the offspring, one parent is almost always heterozygous (Aa) and the other is homozygous recessive (aa). If you see a 3:1 ratio, both parents are usually heterozygous (Aa x Aa).

    Quick study tip: Always identify the recessive phenotype first; because it only has one possible genotype (aa), it serves as your 'anchor' for solving the rest of the Punnett square.

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