Correct Option: A. Allele Detailed Explanation:

1. Definition of Allele: An allele is a variant form of a gene that is located at a specific position (locus) on a chromosome. Genes can have multiple alleles, which can lead to different traits or characteristics in an organism. For example, the gene for flower color in pea plants can have a purple allele and a white allele.
2. How Alleles Work: Each individual inherits two alleles for each gene, one from each parent. These alleles can be:
3. Homozygous: Both alleles are the same (e.g., two purple alleles).
4. Heterozygous: The alleles are different (e.g., one purple allele and one white allele). The combination of these alleles determines the organism's traits.
5. Example of Alleles in Action: Consider the gene that determines flower color in pea plants:
6. Purple allele (P): Dominant
7. White allele (p): Recessive
8. A plant with the genotype PP or Pp will have purple flowers, while a plant with the genotype pp will have white flowers. This illustrates how different alleles can lead to different observable traits.

Why Other Options Are Incorrect:

• B. Chromosome: A chromosome is a long strand of DNA that contains many genes. While chromosomes carry alleles, they are not the variant forms of genes themselves. Instead, they are the structures that house genes and alleles.
• C. Genotype: The genotype refers to the genetic makeup of an organism, specifically the alleles it possesses for a particular gene. While the genotype includes alleles, it is not a variant form of a gene; rather, it is the combination of alleles (e.g., PP, Pp, or pp) that an organism has.
• D. Phenotype: The phenotype is the observable physical or biochemical characteristics of an organism, which result from the interaction of its genotype with the environment. While the phenotype is influenced by alleles, it is not a variant form of a gene. For example, the phenotype could be the color of the flowers (purple or white), but the alleles are the specific versions of the gene that determine that color.

Summary of Key Points:

• Alleles are variant forms of a gene that can produce different traits in an organism.
• Each individual has two alleles for each gene, which can be homozygous or heterozygous.
• The genotype is the combination of alleles, while the phenotype is the observable trait.
• Chromosomes are structures that contain genes and alleles but are not themselves variants of genes.

This understanding of alleles is fundamental in genetics, as it lays the groundwork for studying inheritance patterns, genetic variation, and the principles of Mendelian genetics.