Mendelian terminology refresher: Which term denotes a pair of contrasting hereditary factors that occupy the same locus on homologous chromosomes?

Introduction / Context:
Understanding core genetic vocabulary is vital for solving inheritance problems. Mendel’s “factors” correspond to genes, and the alternative versions that account for contrasting traits (such as tall vs. dwarf) are called alleles.

Given Data / Assumptions:

• Two contrasting forms of a trait are considered (e.g., round vs. wrinkled seeds).
• They occupy the same locus on homologous chromosomes in a diploid organism.

Concept / Approach:
Alleles are alternative forms of the same gene. Phenotype is the observed trait. Genotype is the genetic constitution (e.g., AA, Aa, aa). Homozygous refers to identical alleles at a locus. Haplotype refers to a set of alleles on one chromosome segment transmitted together. Only “alleles” specifically denotes contrasting forms at the same locus.

Step-by-Step Solution:

Verification / Alternative check:
Standard genetics texts define allele as one of two or more alternative forms of a gene at a locus, responsible for contrasting characters.

Why Other Options Are Wrong:

• Phenotype: outward appearance, not the genetic variants themselves.
• Homozygous: condition of having identical alleles, not the alleles.
• Genotype: full genetic makeup, more general than alleles at a single locus.
• Haplotype: linked allele set on one chromosome, not necessarily contrasting forms at one locus.

Common Pitfalls:
Equating genotype with allele or thinking phenotype refers to genes rather than traits.

Final Answer:
Alleles