High School

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

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

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

A. The red geranium is heterozygous for red flowers (RI).
B. The red geranium is homozygous for white flowers (TI).
C. The red geranium is homozygous for red flowers (RR).
D. The trait shows incomplete dominance.

Answer :

Sure, let's solve this step-by-step:

1. Determine Offspring Genotypes:
- The horticulturalist crosses a red geranium with a white geranium resulting in offspring where half have red flowers and half have white flowers. The white geranium has the genotype [tex]\( II \)[/tex].

2. Possible Genetic Makeup of Offspring:
- The offspring show a 50% red and 50% white distribution. This indicates two different genotypes among the offspring: red (Rr) and white (rr).

3. Genotype of Offspring:
- The table of offspring shows:
- Two offspring have the genotype [tex]\( Rr \)[/tex] (red).
- Two offspring have the genotype [tex]\( rr \)[/tex] (white).

4. Deduce Parent Genotypes:
- The red geranium must have produced both [tex]\( Rr \)[/tex] (red) and [tex]\( rr \)[/tex] (white) offspring.
- The white geranium has the genotype (rr) as given.

5. Crossing Analysis:
- If we think about the genetic cross, when a heterozygous red geranium (Rr) is crossed with a homozygous white geranium (rr):
- [tex]\( Rr \)[/tex] (red) crossed with [tex]\( rr \)[/tex] (white) results in:
- 50% [tex]\( Rr \)[/tex] (red)
- 50% [tex]\( rr \)[/tex] (white)

6. Conclusion:
- This 50%-50% distribution matches exactly with the observed offspring.
- Therefore, the red geranium parent must be heterozygous, meaning it has one allele for red flowers (R) and one allele for white flowers (r).

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