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?

Rationale

The unknown genotype is Aa.

In this cross involving an organism exhibiting a recessive trait (aa), the presence of offspring displaying both dominant and recessive traits in the Punnett square indicates that the unknown genotype must be heterozygous (Aa). This allows for the possibility of producing both phenotypes.

A (a) YOUR ANSWER

If the unknown genotype were simply "a," all offspring would inherit "a" from both parents, resulting exclusively in the recessive phenotype (aa). This outcome contradicts the presence of offspring exhibiting dominant traits, thus eliminating "a" as a viable option.

B (Aa) CORRECT

The genotype "Aa" would produce offspring that could express both dominant and recessive traits when crossed with "aa." In this scenario, half of the offspring would show the dominant phenotype (A) while the other half would exhibit the recessive phenotype (aa). This dual outcome aligns with the results observed in the Punnett square, confirming "Aa" as the correct answer.

C (AA) YOUR ANSWER

If the unknown genotype were "AA," all offspring would inherit an "A" from the unknown parent and an "a" from the recessive parent, resulting in all offspring expressing the dominant phenotype (Aa). This outcome does not match the observed results, ruling out "AA" as a possibility.

D (aa) YOUR ANSWER

An unknown genotype of "aa" would yield offspring that are all recessive (aa) when crossed with another "aa" parent. This situation is inconsistent with the presence of any dominant traits in the offspring, thus dismissing "aa" as a potential genotype.

Conclusion

The unknown genotype must be heterozygous (Aa) to account for the varied phenotypic outcomes observed in the offspring when crossed with a recessive organism (aa). This illustrates the basic principles of Mendelian genetics, where the presence of a dominant allele allows for the expression of dominant traits even in the presence of a recessive counterpart.

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?

The unknown genotype is Aa.

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