A horticulturalist wants to produce geraniums with specific characteristics.

Question

She knows that the trait of red flowers is governed by the allele R(RR and Rr) and the trait of white flowers is governed by the allele r (rr).

The horticulturalist has a red geranium with an uncertain genetic makeup. She crosses it with a white geranium (rr) and gets a generation in which all of the plants have red flowers.

r r
? Rr Rr
? Rr Rr
What does this combination of offspring tell her about the genetic makeup of the red geranium parent?

Responses

The red geranium is homozygous for white flowers (rr).
The red geranium is homozygous for white flowers left parenthesis lowercase r r right parenthesis .

The trait shows incomplete dominance.
The trait shows incomplete dominance.

The red geranium is homozygous for red flowers (RR).
The red geranium is homozygous for red flowers left parenthesis uppercase R R right parenthesis .

The red geranium is heterozygous for red flowers (Rr).

Answer 1

The fact that all the offspring from the cross between the red geranium (unknown genetic makeup) and the white geranium (rr) have red flowers indicates that the red geranium must have at least one R allele to produce the red flower phenotype.

Since the offspring produced red flowers (Rr), and the only way to achieve this outcome is if the red geranium is carrying one R allele, we can conclude that the red geranium is heterozygous for red flowers (Rr). If it were homozygous for red flowers (RR), the expected offspring would still have red flowers, but the specific genetic makeup of the parent (since it is unclear) has indeed led us to infer its heterozygosity as the most plausible conclusion, especially as it consistently produced the red flowered offspring.

Thus, the correct answer would be:

The red geranium is heterozygous for red flowers (Rr).