Unknown
a Aa aa
a Aa aa
An organism with an unknown genotype is crossed with another organism that shows the recessive trait (aa). The Punnet square above shows the results. Which of the following is the unknown genotype?
A.
a
B.
Ab
C.
aa
D.
AA
The correct answer and Explanation is :
To determine the unknown genotype in this scenario, let’s analyze the provided options and the information given about the Punnett square. The key point is that one organism has the known recessive phenotype (aa), and we need to identify the genotype of the other organism based on the offspring results.
Explanation:
- Genotypes and Phenotypes:
- In genetics, the uppercase letter (A) represents the dominant allele, while the lowercase letter (a) represents the recessive allele.
- The recessive phenotype (aa) will only manifest if both alleles are recessive (i.e., the organism must have two recessive alleles).
- Crossing with aa:
- When the unknown genotype is crossed with aa, the offspring will show a combination of the alleles from both parents.
- If the unknown genotype is recessive (aa), all offspring will be aa, displaying the recessive trait.
- If the unknown genotype is heterozygous (Aa), approximately half of the offspring will be Aa (dominant trait) and half will be aa (recessive trait).
- Options:
- A. a: This cannot be the unknown genotype since a single “a” would imply that only one recessive allele is present, leading to offspring that can’t solely express the recessive trait.
- B. Ab: This is not a valid genotype as “b” does not relate to the alleles presented (A and a).
- C. aa: If the unknown genotype is aa, then all offspring will be aa, which fits the scenario where the recessive phenotype is expressed.
- D. AA: If the unknown genotype were AA, all offspring would express the dominant phenotype (A), which contradicts the presence of any recessive phenotypes in the offspring.
Conclusion:
Given this analysis, the only viable option that aligns with producing offspring of the recessive trait (aa) is C. aa. This genotype explains the presence of the recessive trait in the offspring, confirming the unknown organism must indeed be homozygous recessive. Thus, the correct answer is C. aa.