In Mendelian genetics, the principle of dominance states which of the following about how alleles of a gene express themselves in a heterozygous individual?

Introduction / Context:
Gregor Mendel formulated several key principles of inheritance while studying pea plants. One of these is the principle of dominance. This principle explains why certain traits appear in the first generation when two different pure breeding parents are crossed. The question asks you to recall the correct statement that summarises this principle in terms of dominant and recessive alleles in a heterozygous individual.

Given Data / Assumptions:

- The context is Mendelian inheritance of a single gene with two alleles. - The focus is on what happens in a heterozygous individual (having two different alleles). - Options describe different possible relationships between alleles. - We assume simple complete dominance rather than incomplete dominance or codominance.

Concept / Approach:
According to the principle of dominance, when two contrasting alleles for a trait are present in an individual, one can mask or dominate the expression of the other in the phenotype. The allele that expresses its effect in the heterozygote is called dominant, and the allele that is masked is called recessive. This is why, in Mendel classic monohybrid crosses, one trait appears in the F1 generation while the other trait seems to disappear, only to reappear in the F2 generation when recessive alleles pair again.

Step-by-Step Solution:
1. Consider a gene with two alleles, such as tall (T) and dwarf (t) in pea plants. 2. When a pure tall plant (TT) is crossed with a pure dwarf plant (tt), the F1 generation inherits one T allele and one t allele, becoming Tt (heterozygous). 3. In these Tt plants, the tall phenotype appears, and the dwarf trait is not visible, even though the t allele is present. 4. This shows that the tall allele (T) is dominant and masks the expression of the dwarf allele (t), which is recessive. 5. Mendel generalised this observation, stating that some alleles are dominant while others are recessive, and that the dominant allele determines the phenotype in a heterozygote. 6. Therefore, the principle of dominance is correctly described by the idea that some alleles can mask the expression of others when both are present in the same individual.

Verification / Alternative check:
Experimental data from Mendel various monohybrid crosses demonstrate that only one of the two parental traits appears in the F1 generation, even though both alleles are present. The hidden trait reappears in a 3:1 ratio in the F2 generation when alleles segregate and recombine. This pattern strongly supports the principle of dominance and recessiveness for many simple traits. Later molecular studies have shown that dominant alleles often produce functional proteins, while recessive alleles may represent loss of function variants, offering a mechanistic explanation.

Why Other Options Are Wrong:
- All alleles are recessive and only show their effect when paired together: This contradicts the appearance of dominant traits in heterozygotes and does not explain the F1 generation observations. - All alleles are dominant and always express fully, regardless of pairing: If this were true, heterozygotes would show a mixture of traits, not a single dominant phenotype. - Alleles are neither dominant nor recessive and always blend equally: This describes a blending inheritance model, which Mendel principles and observed ratios clearly disproved for many traits.

Common Pitfalls:
A frequent misunderstanding is thinking that dominance means an allele is more common or better. Dominant alleles are not necessarily more frequent or more beneficial; dominance simply describes how alleles interact to determine phenotype. Another pitfall is to assume all traits follow simple dominance, when in reality incomplete dominance, codominance, and polygenic inheritance also occur. However, for classic Mendelian monohybrid traits, the principle of dominance accurately captures how some alleles mask others in heterozygotes.

Final Answer:
The principle of dominance states that some alleles are dominant and can mask the expression of other alleles that are recessive in a heterozygous individual.