Model Answer
0 min readIntroduction
Gregor Mendel's laws of inheritance form the foundation of modern genetics. The concept of dominance, where one allele masks the expression of another, is central to understanding inheritance patterns. Determining the genotypes of parents based on the phenotypes of their offspring requires careful analysis of the observed ratios and application of Punnett square methodology. This question assesses the ability to apply these principles to a classic genetic problem involving seed color in peas, a model system used extensively by Mendel himself. The problem requires us to deduce the parental genotypes based on the progeny phenotypes.
Understanding the Problem
The problem states that gray seed color (G) is dominant over white seed color (g). This means that individuals with either GG or Gg genotype will exhibit the gray phenotype, while only individuals with the gg genotype will exhibit the white phenotype. We are given progeny data and need to determine the genotypes of the parents.
Analyzing the Progeny Data
Let's assume the progeny data is as follows (this data is missing from the original question, so I'm creating a plausible scenario for demonstration. The answer will be adjusted based on the actual data if provided):
- Gray seeds: 270
- White seeds: 90
This gives us a phenotypic ratio of 3:1 (Gray:White). This ratio is characteristic of a monohybrid cross involving two heterozygous parents.
Determining Parental Genotypes
To confirm this, let's construct a Punnett square for a cross between two heterozygous parents (Gg x Gg):
| G | g | |
|---|---|---|
| G | GG | Gg |
| g | Gg | gg |
From the Punnett square, we get the following genotypic ratio:
- GG: 1/4
- Gg: 2/4 (or 1/2)
- gg: 1/4
And the following phenotypic ratio:
- Gray (GG + Gg): 3/4
- White (gg): 1/4
This matches the observed 3:1 phenotypic ratio in the progeny. Therefore, the genotypes of the parents are both Gg (heterozygous).
Alternative Scenarios (If different progeny data is provided)
If the progeny data showed a 1:1 ratio (e.g., 100 Gray, 100 White), it would suggest a cross between a heterozygous (Gg) and a homozygous recessive (gg) parent. If all progeny were gray, it would suggest either GG x GG, GG x Gg, or Gg x Gg. Further generations would be needed to determine the exact genotypes in these cases.
Importance of Sample Size
It's important to note that the accuracy of genotype determination relies on a sufficiently large sample size. A small sample size can lead to deviations from the expected ratios due to chance.
Conclusion
Based on the assumed progeny data of a 3:1 gray to white seed ratio, the genotypes of both parents are determined to be heterozygous (Gg). This conclusion is derived from applying Mendelian principles of dominance and segregation, and utilizing the Punnett square to visualize the possible genetic combinations. Accurate genotype determination is crucial for understanding inheritance patterns and predicting the characteristics of future generations. The validity of this conclusion depends on the accuracy and size of the observed progeny data.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.