In classical genetics, a pair of contrasting forms of the same gene that control a single trait are called what?

Introduction / Context:
Gregor Mendel's experiments with pea plants introduced key concepts in heredity, such as dominant and recessive forms of traits. In modern genetics, these different forms of a gene that affect the same characteristic are given a specific name. Understanding this term is fundamental when studying inheritance patterns, Punnett squares, and genetic crosses. This question asks you to identify what such contrasting forms are called.

Given Data / Assumptions:

• The focus is on a pair of contrasting forms controlling the same trait, such as tall versus dwarf plant height.
• These forms are different versions of the same gene.
• Options include factors, loci, alleles, lineage, and genotype.
• We assume basic knowledge of Mendelian genetics and gene terminology.

Concept / Approach:
A gene is a segment of DNA that controls a particular trait. Different versions of the same gene, which may produce different observable forms of the trait, are called alleles. For example, in pea plants there can be an allele for tall height and an allele for dwarf height. These alleles occupy the same position, or locus, on homologous chromosomes. Mendel originally referred to them as factors, but the modern term allele, or allelomorph, is widely used in genetics. Lineage refers to ancestry, and genotype describes the overall genetic makeup, not the individual alternative forms of a single gene.

Step-by-Step Solution:
Step 1: Identify the key phrase in the question: contrasting forms of the same gene controlling one trait. Step 2: Recall that such different forms, like tall versus dwarf in pea plants, are called alleles. Step 3: Understand that alleles are located at the same locus on homologous chromosomes. Step 4: Note that loci (plural of locus) refer to positions on chromosomes, not the different forms themselves. Step 5: Recognise that factors was Mendel's older term, but modern genetics uses allele for precise description. Step 6: Lineage and genotype refer to broader concepts of ancestry and full genetic constitution, not to single gene forms. Step 7: Therefore, the correct term for contrasting forms of the same gene is alleles.

Verification / Alternative check:
Genetics textbooks define an allele as one of two or more alternative forms of a gene that arise by mutation and are found at the same locus on a chromosome. They use examples like A and a to represent dominant and recessive alleles. Punnett square problems repeatedly refer to alleles rather than factors or loci. This consistent usage confirms that alleles is the correct answer.

Why Other Options Are Wrong:
Factors: An older term used by Mendel, but the question uses modern wording and expects the term alleles. Loci: These are positions on chromosomes where genes are located; they do not refer to the alternative forms. Lineage: Refers to ancestral line or family history, not to individual gene variants. Genotype: The complete set of genes or alleles an organism possesses, not a single pair of contrasting forms.

Common Pitfalls:
Some learners may confuse allele, locus, and gene because they are related concepts. A helpful way to remember is that the locus is the address on the chromosome, the gene is the information stored there, and the allele is a particular version of that information. Keeping these distinctions clear makes it easier to interpret genetic problems correctly.

Final Answer:
The correct answer is Alleles.