Final answer:
In a heterozygous individual, the allele that is expressed is the dominant allele, as it masks the presence of the recessive allele in the phenotype.
Step-by-step explanation:
In a heterozygous individual the allele being expressed is dominant. This is because a dominant allele masks the presence of a recessive allele when the two occur together in a heterozygote. This means that the physical trait associated with the dominant allele will be observed in the organism, while the trait associated with the recessive allele will not be seen unless the organism has two copies of the recessive allele (in other words, is homozygous recessive).
The dominant allele is referred to as the expressed unit factor, while the recessive allele is considered the latent unit factor. This pattern of inheritance is part of Mendel's law of dominance. In heterozygous organisms, dominant and recessive alleles lead to different observable characteristics (phenotypes), but only the dominant trait will be expressed in the phenotype while the recessive trait remains latent unless paired with another recessive allele.