Which statement best describes alleles?

• A. Chromosome segments
• B. Different versions of the same gene
• C. Pairs of matching chromosomes
• D. Physical traits

Answer: (B)

Alleles are different versions of the same gene that occupy the same place on homologous chromosomes. In each cell you have two alleles for every gene—one inherited from each parent. If the two alleles are the same, you’re homozygous for that gene; if they’re different, you’re heterozygous. The way these variants interact determines the trait you show, through patterns like dominance or codominance. For example, a gene for a particular color may have a dominant version and a recessive version, so having at least one dominant allele can produce the dominant color, while two recessive alleles produce the recessive color. The ABO blood type system is another illustration, where different alleles combine to yield A, B, AB, or O types. This shows that alleles are not chromosome segments, nor pairs of matching chromosomes themselves, and they are not traits by themselves but the variant forms that shape traits.