Question

if two parents with dominant phenotypes produce an offspring with a recessive phenotype, then probably
a only one parent is heterozygous.
b one parent is homozygous recessive.
c both parents are heterozygous.
d both parents are homozygous dominant.
e both parents are homozygous recessive.

Explanation:

To have a recessive phenotype in offspring, the offspring must have two recessive alleles (genotype \( rr \) for a single - gene trait, for example). For parents with dominant phenotypes, a dominant phenotype can be either homozygous dominant (\( RR \)) or heterozygous (\( Rr \)).

• Option A: If only one parent is heterozygous (\( Rr \)) and the other is homozygous dominant (\( RR \)), the possible genotypes of the offspring are \( RR \) and \( Rr \), both of which show the dominant phenotype. So, it's not possible to get a recessive offspring in this case.
• Option B: If one parent is homozygous recessive (\( rr \)) and the other has a dominant phenotype. If the dominant - phenotype parent is homozygous dominant (\( RR \)), the offspring will be \( Rr \) (dominant phenotype). If the dominant - phenotype parent is heterozygous (\( Rr \)), there is a chance of getting \( rr \) (recessive phenotype), but this option doesn't guarantee that the dominant - phenotype parent is heterozygous. So, this is not the most probable case.
• Option C: If both parents are heterozygous (\( Rr\times Rr \)), the Punnett square shows that the genotypic ratio is \( 1RR: 2Rr: 1rr \). So, there is a \( 25\% \) chance of getting an offspring with the recessive phenotype (\( rr \)). This is a valid way to get a recessive offspring from two dominant - phenotype parents (since heterozygous parents have a dominant phenotype).
• Option D: If both parents are homozygous dominant (\( RR\times RR \)), all offspring will be \( RR \), showing the dominant phenotype. So, no recessive offspring can be produced.
• Option E: If both parents are homozygous recessive (\( rr\times rr \)), all offspring will be \( rr \) (recessive phenotype), but the parents here have a recessive phenotype, not a dominant phenotype as stated in the question (the question is about two parents with dominant phenotypes).

So, the most probable situation is that both parents are heterozygous.

Answer:

C. both parents are heterozygous.