Which of the following is true regarding genotype to phenotype expression?

Question

BlackTom AI · Accepted Answer

For a single-gene trait with two alleles (A and a), Mendelian segregation produces four equally possible allele combinations in the offspring when both parents are heterozygous (Aa × Aa).

1. Relevant knowledge points and concepts
- Alleles: alternative forms of a gene (A and a).
- Genotype vs phenotype: genotype is the allele pair (AA, Aa, aa); phenotype is the observable trait determined by how those alleles are expressed (dominant, recessive, codominant, incomplete dominance).
- Punnett square and probability: each parent produces gametes carrying A or a with probability 1/2 when heterozygous.

2. Step-by-step reasoning
- Parental genotypes: Aa (mother) × Aa (father).
- Gametes from each parent: 1/2 A and 1/2 a.
- Constructing the 2×2 Punnett square yields four boxes representing all allele pairings.

3. Calculations and intermediate steps
- The four Punnett-square outcomes (each with probability 1/4) are:
  1) A (from parent1) × A (from parent2) → AA : 1/4
  2) A × a → Aa : 1/4
  3) a × A → aA : 1/4
  4) a × a → aa : 1/4
- Combining equivalent genotypes: AA = 1/4, Aa or aA combined = 1/2, aa = 1/4. These translate to phenotype probabilities depending on dominance relationships.

4. Why the correct answer is correct
- Option C states that two heterozygous parents will produce an offspring who has one of four phenotypic outcomes. The Punnett square explicitly shows four equally likely allele-pair outcomes (four boxes). Each box corresponds to an offspring outcome (genotypic and, depending on expression, phenotypic outcome). Thus, for the purposes of inheritance outcomes enumerated by the Punnett square, it is correct to say one of four possible outcomes occurs.

5. Why the other options are incorrect
- A: Codominance expresses both parental phenotypes simultaneously (e.g., AB blood type), not a completely novel phenotype; incomplete dominance can give an intermediate phenotype, but codominance does not create a wholly new trait.
- B: Most sex-linked recessive disorders are X-linked; males inherit X-linked recessive alleles from their mother, not exclusively via the Y chromosome. Y-linked disorders are rare and transmitted father-to-son.
- D: Not all phenotypes are polygenic; many traits are controlled by single genes (Mendelian inheritance).

6. Conclusion
The correct choice is C) Two heterozygous parents will produce an offspring who has one of four phenotypic outcomes, which matches the provided answer.

Which of the following is true regarding genotype to phenotype expression?Single choice

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