In a certain plant, red flowers are dominant over white flowers. A plant heterozygous for red flowers and a plant with white flowers are crossed. Which of the following is the expected proportion of phenotypes in the next generation?
1 red, 3 white
red, 0 white
red, 1 white
2 red, 2 white
The Correct Answer and Explanation is:
Correct Answer: 2 red, 2 white
Explanation:
This question is based on basic Mendelian genetics, particularly the concept of dominant and recessive alleles.
Key Genetic Concepts:
- Alleles are different forms of a gene. In this case:
- R = dominant allele for red flowers
- r = recessive allele for white flowers
- Genotypes represent the combination of alleles in an organism.
- RR = Homozygous dominant (red flowers)
- Rr = Heterozygous (red flowers, because red is dominant)
- rr = Homozygous recessive (white flowers)
Cross Description:
- One parent is heterozygous red (Rr)
- The other parent has white flowers (rr)
We use a Punnett square to determine the offspring:
| r (white parent) | r (white parent) | |
|---|---|---|
| R (red parent) | Rr (red) | Rr (red) |
| r (red parent) | rr (white) | rr (white) |
Resulting Genotypes:
- 2 Rr = red-flowered plants
- 2 rr = white-flowered plants
This simplifies to a 1:1 ratio or 2 red : 2 white, which is a 50% chance for each flower color in the next generation.
Phenotypic Ratio (Observable Traits):
- 50% red flowers
- 50% white flowers
This is because the red-flowered offspring (Rr) have one dominant allele, making red visible. The white-flowered offspring (rr) lack the dominant allele and thus express the recessive white flower trait.
Incorrect Choices Explained:
- 1 red, 3 white: Would imply a 1:3 ratio, which doesn’t match a Rr × rr cross.
- red, 0 white: Incorrect, since rr offspring clearly produce white flowers.
- red, 1 white: Vague and incomplete—doesn’t express a clear ratio.
Conclusion:
The expected proportion of phenotypes in this cross between a heterozygous red-flowered plant and a white-flowered plant is:
2 red, 2 white or 1:1 ratio.