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Practice Question

Punnet SquareAn organism with an unknown genotype is crossed with another organism that shows the recessive trait (aa). The Punnet square below shows the results. Which of the following is the unknown genotype?

Answer Choices:

Correct Answer:

Aa

Rationale:

🧬The Punnett square shows that the offspring consist of two Aa (heterozygous) and two aa (homozygous recessive) genotypes.

🧪This 1:1 ratio of dominant-to-recessive phenotypes suggests that the unknown parent must be heterozygous (Aa), contributing either an A or a to each offspring.

🔬If the unknown parent were AA, all offspring would have been Aa, and none would have the recessive aa trait.

👩‍🔬The presence of aa offspring proves that the unknown genotype must carry the recessive allele, ruling out both AA and aa as possible genotypes.

📌Core Message

The unknown genotype must be heterozygous (Aa) because crossing it with a recessive (aa) organism produced a 1:1 ratio of heterozygous (Aa) and homozygous recessive (aa) offspring.

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This question is from SCIENCE 2023 Exam-3 which contains 49 questions.

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Question Details
  • Category: ATI TEAS
  • Subcategory: SCIENCE
  • Domain: SCIENCE 2023
  • Answer Choices: 4
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