She knows that the trait of red flowers is governed by the allele [tex]$ R $[/tex] ([tex]$ RR $[/tex] and [tex]$ Rr $[/tex]), and the trait of white flowers is governed by the allele [tex]$ r $[/tex] ([tex]$ rr $[/tex]).

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

[tex]\[
\begin{tabular}{|c|c|c|}
\hline
& r & r \\
\hline
? & Rr & Rr \\
\hline
? & rr & rr \\
\hline
\end{tabular}
\][/tex]

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

A. The red geranium is homozygous for red flowers ([tex]$ RR $[/tex]).

B. The red geranium is heterozygous for red flowers ([tex]$ Rr $[/tex]).

C. The red geranium is homozygous for white flowers ([tex]$ rr $[/tex]).

To determine the genetic makeup of the red geranium parent, we can analyze the offspring from the cross with a white geranium.

1. Traits Description:
- Red flowers appear if the plant has either two dominant alleles (RR) or one dominant and one recessive allele (Rr).
- White flowers appear only if the plant has two recessive alleles (rr).

2. Cross Analysis:
- The white geranium used for the cross is homozygous recessive (rr).

3. Offspring Results:
- The offspring consists of half red-flowered plants and half white-flowered plants.

4. Genetic Interpretation:
- For the offspring to have both red and white flowers, the red parent must have contributed one dominant R allele to some offspring (resulting in red flowers) and one recessive r allele to the others (resulting in white flowers).
- This indicates that the red geranium parent must be heterozygous (Rr). Only with this genotype (Rr), when crossed with a white geranium (rr), can you get:
- 50% offspring with Rr genotype (red flowers)
- 50% offspring with rr genotype (white flowers)

Thus, the genetic makeup of the red geranium parent is heterozygous for red flowers, Rr.