Which term describes an allele expressed when present on only one of the two homologous chromosomes in a diploid organism?

Which term describes an allele expressed when present on only one of the two homologous chromosomes in a diploid organism?

• A. Dominant
• B. Recessive
• C. Co-dominant
• D. Incomplete dominance

Answer: (A)

Dominant alleles are expressed when present in just one copy in a diploid organism because the product of that single allele is enough to determine the phenotype, often masking the other allele. In a heterozygote, the dominant allele's effect shows up even if the second chromosome carries a different allele, so you see the dominant trait. The recessive allele would only determine the phenotype if both chromosomes carry it. Co-dominance and incomplete dominance describe different patterns where either both alleles are fully expressed or the heterozygote shows an intermediate phenotype, not a single allele dictating the trait in a heterozygote.

Dominant alleles are expressed when present in just one copy in a diploid organism because the product of that single allele is enough to determine the phenotype, often masking the other allele. In a heterozygote, the dominant allele's effect shows up even if the second chromosome carries a different allele, so you see the dominant trait. The recessive allele would only determine the phenotype if both chromosomes carry it. Co-dominance and incomplete dominance describe different patterns where either both alleles are fully expressed or the heterozygote shows an intermediate phenotype, not a single allele dictating the trait in a heterozygote.