Question

If the dominant phenotype parent of unknown
genotype produces only dominant phenotype
offspring in a testcross, the parent was: (answer
by writing out the testcross).
Use letter A as the dominant allele, and letter a
as the recessive allele.

Answer 1

Final answer:

The parent was homozygous dominant (AA).

Explanation:

In a testcross, the individual of unknown genotype is crossed with a homozygous recessive individual. If the dominant phenotype parent of unknown genotype produces only dominant phenotype offspring in the testcross, it implies that the unknown parent must be homozygous dominant (AA). This is because the only way for all offspring to exhibit the dominant phenotype is if the parent passes on the dominant allele (A) to all offspring, which can only occur if the parent is homozygous dominant.

This conclusion can be verified through the Punnett square. In a testcross between the unknown parent and a homozygous recessive parent (aa), the possible genotypes of the unknown parent are Aa and AA. However, if all offspring display the dominant phenotype, the unknown parent must be AA. This is because if the unknown parent were Aa, there would be a 50% chance for each offspring to inherit the recessive allele (a) and exhibit the recessive phenotype, which contradicts the observation that all offspring exhibit the dominant phenotype. Therefore, the unknown parent in this scenario is homozygous dominant (AA).