Final answer:
A dominant option (a) allele will be expressed in the organism's phenotype whenever it is present, whether the organism is homozygous dominant or heterozygous. A recessive allele is only expressed phenotypically when the organism is homozygous recessive, carrying two copies of the recessive allele.
Step-by-step explanation:
An allele that exerts its effects whenever it is present is known as a dominant allele. This means if an organism has at least one dominant allele for a gene, the effect of that allele will be visible in the organism's phenotype.
In contrast to a recessive allele, a dominant allele will be expressed in both; organisms that are homozygous for the dominant allele (carrying two copies of the dominant allele), and heterozygous organisms (carrying one dominant allele and one recessive allele).
A recessive trait, on the other hand, will only be observed in individuals who are homozygous for the recessive trait, meaning they have two copies of the recessive allele. In the case of someone with one recessive and one dominant allele, the dominant trait will be expressed, showing the dominant phenotype.
In conclusion, Mendel's theory demonstrates that for a gene with one dominant and one recessive allele, the phenotypic expression will be that of the dominant trait unless the individual is homozygous for the recessive allele, in which case the recessive trait is expressed.