Law of Dominance describes which of the following?

• A. When an organism carries two different alleles, both are expressed.
• B. When identical alleles combine to form a trait
• C. Alleles always segregate independently
• D. Dominant alleles are only expressed in the presence of recessive alleles

Answer: (B)

Dominance is about how one allele can mask the effect of another in the organism’s visible traits. When an organism has two different alleles for a trait, the dominant allele determines the phenotype, so the recessive allele isn’t seen unless both copies are recessive. For example, in pea plants, the allele for purple flowers is dominant over white; a plant with Purple/White or Purple/Purple shows purple flowers, while only White/White shows white.

The idea described as two identical alleles forming a trait is just homozygosity, not about dominance. The idea that alleles always segregate independently recalls independent assortment, not dominance. And the notion that dominant alleles are only expressed when a recessive is present is the opposite of how dominance works—the dominant trait appears even when paired with a recessive.