1. When an organism with an unknown genotype is crossed with another organism displaying the recessive trait (aa), the Punnett square shown above illustrates the outcomes. What is the unknown genotype?
Answer: B
The unknown genotype is Aa.
The unknown genotype must be heterozygous (Aa) since crossing it with a homozygous recessive organism (aa) results in offspring that display both dominant and recessive traits, confirming the presence of the dominant allele.
A) a
This option represents a homozygous recessive genotype (aa), which would not produce any offspring displaying the dominant trait. Therefore, it cannot be the unknown genotype since it would only yield offspring with the recessive trait.
B) Aa
This option is correct because crossing an organism with the genotype Aa with an organism of genotype aa would result in a 1:1 ratio of offspring exhibiting the dominant trait (A) and the recessive trait (a). This confirms that the unknown genotype is indeed heterozygous (Aa).
C) AA
This option indicates a homozygous dominant genotype. If the unknown genotype were AA, all offspring produced from a cross with aa would display the dominant phenotype, which contradicts the presence of any recessive traits in the offspring.
D) aa
This option suggests a homozygous recessive genotype. If the unknown genotype were aa, all offspring would be aa and exhibit the recessive trait, which is not consistent with the observed outcomes in the Punnett square.
Conclusion
The correct answer, Aa, is confirmed by the offspring ratios produced in the cross with aa, which allow for both dominant and recessive traits to appear. All other options fail to provide a valid explanation for the presence of both traits among the offspring, making them incorrect.