Question

The Rocky Mountain gardener found that crossing wrinkle-seeded plants with pure round-seeded plants produced only round-seeded plants. What genotypic & phenotypic ratios can be expected from a cross of a wrinkle-seeded plant & a plant heterozygous for this trait (seed appearance)?

Answer 1

In this scenario, we are dealing with a classic example of Mendelian inheritance in pea plants, where seed shape is a trait determined by a single gene with two alleles: round (R) and wrinkled (r). Round seeds (R) are dominant over wrinkled seeds (r).

1. **Genotypes:**
- The round-seeded plant can be either homozygous (RR) or heterozygous (Rr).
- The wrinkle-seeded plant will be homozygous recessive (rr).

Since the first cross produced only round-seeded plants, we can conclude that the round-seeded plant used in that cross was homozygous (RR).

Now, when crossing a wrinkle-seeded plant (rr) with a heterozygous round-seeded plant (Rr), we set up a Punnett square to analyze the potential genotypic outcomes:

- Parent genotypes:
- Round-seeded plant (heterozygous): Rr
- Wrinkle-seeded plant: rr

2. **Punnett Square:**

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

3. **Results:**
- The possible genotypes from this cross are:
- Rr (round seeds) - 2 instances
- rr (wrinkled seeds) - 2 instances

4. **Ratios:**
- **Genotypic ratio:** 2 Rr : 2 rr (which simplifies to 1 Rr : 1 rr)
- **Phenotypic ratio:** 2 round-seeded : 2 wrinkled (which simplifies to 1 round-seeded : 1 wrinkled)

In conclusion, when a wrinkle-seeded plant is crossed with a heterozygous plant (Rr), the expected genotypic ratio is 1 Rr : 1 rr, and the expected phenotypic ratio is 1 round-seeded : 1 wrinkled.