Dominant trait expresses in:(1994)
**Core Concept**
The question is asking about the expression of a dominant trait, which is a fundamental concept in genetics. A dominant trait is a characteristic that will always be expressed if an individual has one copy of the dominant allele, regardless of the presence of the recessive allele. This is due to the Mendelian law of dominance, which states that a dominant allele will mask the effect of a recessive allele.
**Why the Correct Answer is Right**
The correct answer is related to the genotype of an individual. According to the Mendelian law of dominance, a dominant allele (represented by a capital letter) will always be expressed if an individual has one copy of the dominant allele. For example, if the dominant allele is represented by the letter "B" and the recessive allele is represented by the letter "b", an individual with the genotype "Bb" will express the dominant trait. This is because the dominant allele "B" will mask the effect of the recessive allele "b".
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the genotype of the individual. The expression of a dominant trait depends on the genotype of the individual, not just the presence of the dominant allele.
* **Option B:** This option is incorrect because it implies that the dominant trait is expressed only when the individual has two copies of the dominant allele. However, according to the Mendelian law of dominance, a dominant allele will always be expressed if an individual has one copy of the dominant allele.
* **Option C:** This option is incorrect because it implies that the recessive allele is dominant. However, according to the Mendelian law of dominance, the recessive allele will always be masked by the dominant allele.
**Clinical Pearl / High-Yield Fact**
In genetics, the expression of a dominant trait is a key concept that can be applied to various medical conditions. For example, sickle cell anemia is a dominant trait that can cause anemia and other complications. Understanding the Mendelian law of dominance is crucial for genetic counseling and diagnosis of genetic disorders.
**Correct Answer:** D.