A horticulturalist wants to produce geraniums with specific characteristics.

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

The horticulturalist crosses two geraniums from the F1 generation. The Punnett square demonstrates this cross:

[tex]
\[
\begin{array}{|c|c|c|}
\hline
& R & r \\
\hline
R & RR & Rr \\
\hline
r & Rr & rr \\
\hline
\end{array}
\]
[/tex]

Which ratio of red- to white-flowering plants would she expect to see in the phenotypes of the F2 generation?

A. 1 red: 3 white
B. 3 red: 1 white
C. 2 red: 2 white
D. all red

We start by recognizing that the red flower trait is dominant and the white flower trait is recessive. In the F1 generation, each plant has the heterozygous genotype [tex]$Rr$[/tex]. When two [tex]$Rr$[/tex] plants are crossed, their gametes (each carrying either [tex]$R$[/tex] or [tex]$r$[/tex]) combine according to the following Punnett square:

[tex]$$
\begin{array}{c|cc}
& R & r \\\hline
R & RR & Rr \\
r & Rr & rr \\
\end{array}
$$[/tex]

This square shows the following genotypes among the offspring (F2 generation):

- [tex]$RR$[/tex] (red) occurs once.
- [tex]$Rr$[/tex] (red) occurs twice.
- [tex]$rr$[/tex] (white) occurs once.

Since both [tex]$RR$[/tex] and [tex]$Rr$[/tex] result in red flowers (because [tex]$R$[/tex] is dominant), there are [tex]$3$[/tex] plants with red flowers and [tex]$1$[/tex] plant with white flowers.

Thus, the ratio of red-flowering plants to white-flowering plants in the F2 generation is:

[tex]$$
3 : 1
$$[/tex]