Both recesssive and dominant characters are found

Analyze the question

The question asks where recessive and dominant characters (alleles) are found. In genetics, a dominant allele and a recessive allele of the same gene are alternative forms of that gene (a pair of alleles).

Explain the concept

• A gene is a unit of heredity found at a specific position on a chromosome, called a locus (plural: loci).
• Alleles are alternative forms of the same gene. They occupy the same locus on homologous chromosomes, one on each chromosome of the pair.
• In a heterozygous individual (e.g. Tt), the dominant allele (T) is on one homologous chromosome and the recessive allele (t) is at the same locus on the other. The dominant allele is expressed, while the recessive one is masked.

Solution

• A. On different chromosomes in the cell: Genes on non-homologous chromosomes are different genes (this relates to independent assortment), not dominant and recessive alleles of the same trait. Wrong.
• B. At the same locus of a homologous chromosome: Matches the definition of alleles. Correct.
• C. At different loci of a homologous chromosome: Different loci carry different genes, not contrasting alleles of one trait. Wrong.
• D. On the same chromatid in a chromosome: A single chromatid carries only one allele at any given locus, so it cannot hold both a dominant and a recessive allele of the same gene. Wrong.

Exam tip: Remember "Alleles = Same locus, Homologous chromosomes." Whenever you see a question on contrasting characters (tall/short, round/wrinkled), think of a pair of alleles sitting at the same position on a pair of homologous chromosomes. Also note the spelling in the question: "logus" means "locus."