When can a dominant trait be observed in an organism?

A dominant trait can be observed in an organism when at least one dominant allele is present. This is fundamental to understanding the principles of inheritance described by Mendel. A dominant allele expresses its phenotype even in the presence of a recessive allele. Therefore, an organism with one dominant allele and one recessive allele will exhibit the trait associated with the dominant allele.

For example, if the allele for tall plants (T) is dominant over the allele for short plants (t), both TT (homozygous dominant) and Tt (heterozygous) plants will be tall, while only tt (homozygous recessive) plants will be short. Thus, the presence of just one dominant allele is sufficient for the dominant trait to manifest in the organism’s phenotype.

This concept is crucial to genetics and helps explain why some traits appear more frequently in populations, as dominant alleles can suppress the expression of recessive alleles.