Which one is exception to Mendel's principle of dominance ?

Options

A. Wild Pea
B. Mirabilis
C. Garden Pea
D. Maize

Answer

**Step-by-Step Solution:**

1. **Understanding Mendel's Principle of Dominance**: Mendel's principle of dominance states that in a heterozygous individual, one allele (dominant) can mask the expression of another allele (recessive). This means that only the dominant trait will be expressed in the phenotype.

2. **Identifying Exceptions**: While Mendel's principles apply to many traits, there are exceptions where the dominance is not complete. The two main exceptions are:
- **Incomplete Dominance**: This occurs when the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes. For example, in snapdragon flowers, crossing red-flowered (RR) and white-flowered (rr) plants results in pink-flowered (Rr) offspring.
- **Co-Dominance**: In this case, both alleles in a heterozygote are fully expressed, resulting in offspring with a phenotype that shows both traits simultaneously. An example is seen in the ABO blood group system, where both A and B alleles are expressed in individuals with AB blood type.

3. **Specific Example**: The question asks for a specific example that is an exception to Mendel's principle of dominance. One notable example is the plant *Mirabilis jalapa*, commonly known as the four o'clock flower. In this plant, when red-flowered (RR) and white-flowered (rr) plants are crossed, the resulting offspring (Rr) exhibit a pink phenotype, demonstrating incomplete dominance.

4. **Conclusion**: Therefore, the correct answer to the question is *Mirabilis jalapa*, which is an example of incomplete dominance and thus an exception to Mendel's principle of dominance.

**Final Answer**: The exception to Mendel's principle of dominance is *Mirabilis jalapa* (option B).

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