MoCA Science Practice Test 2025 – The All-in-One Guide to Mastering Your Exam!

Dominant alleles are characterized by their ability to express a specific trait in an organism even when only one copy of that allele is present. This means that in a heterozygous genotype, where an organism has one dominant allele and one recessive allele, the dominant allele will determine the phenotype, or observable characteristics, of the organism.

For example, in a gene that controls flower color, if the allele for red flowers is dominant and the allele for blue flowers is recessive, a plant with one red allele and one blue allele will still have red flowers due to the dominance of the red allele. This trait being expressed with just one copy present is the hallmark of dominant alleles.

In contrast, recessive alleles require two copies—one from each parent—to manifest their associated traits in the phenotype. Thus, only when an organism is homozygous for the recessive allele (having two copies) will the recessive trait be expressed.

The other choices do not accurately define the concept of dominant versus recessive alleles. For instance, the idea that dominant alleles are "always beneficial" is misleading, as the effects of alleles can range from neutral to harmful, depending on the context and environment. Moreover, the