Final answer:
When an organism has a dominant and recessive allele, the dominant trait is the one that is observed. This is due to the dominant trait masking or overriding the recessive trait, as per Mendelian inheritance rules.
Step-by-step explanation:
When there is a combination of a dominant and a recessive trait in the alleles, the trait that will be observed or seen is the dominant trait. This is due to the fact that the dominant traits override or mask the presence of recessive traits. It's all about how the rules of Mendelian inheritance work. For instance, let's take an example where brown eyes (B) are dominant over blue eyes (b). If an individual has a genotype of Bb, the phenotype, or physical trait you would see, would be brown eyes because the brown eye allele is dominant over the blue eye allele.
Learn more about Dominant and Recessive Traits