Model Answer
0 min readIntroduction
Mendel's laws of inheritance form the cornerstone of modern genetics. The principle of dominance states that in a heterozygote, one allele will mask the expression of another. Understanding this principle is crucial for predicting the outcome of genetic crosses. This question assesses the ability to apply this principle in reverse – to determine the genotypes of parents based on the phenotypes of their offspring. The Phaseolus bean, with its easily observable seed coat color, provides a suitable model for demonstrating these genetic principles. We will analyze each experiment individually to deduce the parental genotypes.
Understanding the Genetic Notation
Let 'B' represent the allele for dark brown seed coat color (dominant) and 'b' represent the allele for light brown seed coat color (recessive). Therefore:
- BB: Dark brown seed coat
- Bb: Dark brown seed coat (heterozygote)
- bb: Light brown seed coat
Analyzing the Experiments
Experiment 1: Progeny – All Dark Brown
If all progeny are dark brown, the parents could be BB x BB, BB x Bb, or Bb x Bb. However, if the progeny are *all* dark brown, BB x bb is impossible. If the parents were Bb x Bb, we'd expect a 3:1 ratio. Therefore, the most likely genotypes are:
- Parent 1: BB
- Parent 2: BB or Bb
Experiment 2: Progeny – 3 Dark Brown : 1 Light Brown
This classic Mendelian ratio (3:1) indicates a heterozygous cross. The parents must both be heterozygous.
- Parent 1: Bb
- Parent 2: Bb
Experiment 3: Progeny – All Light Brown
For all progeny to be light brown (bb), both parents must be homozygous recessive.
- Parent 1: bb
- Parent 2: bb
Experiment 4: Progeny – 1 Dark Brown : 1 Light Brown
This 1:1 ratio suggests a cross between a homozygous dominant and a heterozygous individual.
- Parent 1: BB
- Parent 2: bb
Alternatively, it could be a cross between BB and Bb, but the probability of this is lower.
Experiment 5: Progeny – 2 Dark Brown : 1 Light Brown
This ratio is not a standard Mendelian ratio. It suggests a backcross, where a heterozygote (Bb) is crossed with a recessive homozygote (bb). The expected ratio is 1:1, but with a larger sample size, it can appear as approximately 2:1.
- Parent 1: Bb
- Parent 2: bb
It's important to note that these are the *most likely* genotypes based on the given information. Larger sample sizes of progeny would provide more definitive results.
Conclusion
Determining parental genotypes from progeny phenotypes relies on a solid understanding of Mendelian inheritance principles, particularly the concepts of dominance and segregation. By carefully analyzing the observed ratios and applying Punnett squares, we can deduce the most probable genotypes of the parents. While some experiments may have multiple possible solutions, the most likely scenarios are based on the expected Mendelian ratios. This exercise highlights the power of genetic analysis in understanding inheritance patterns.
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.