Final answer:
If a person has one copy of each of two different alleles for a given gene, they are heterozygous for that trait, which typically results in the expression of the dominant trait in their phenotype.
Step-by-step explanation:
If a person has one copy of each of two different alleles for a given gene, the person is heterozygous for that trait. Being heterozygous means that the individual has one dominant allele and one recessive allele for the gene in question. In such cases, the dominant trait is typically expressed in the phenotype, making the individual phenotypically similar to a person with two dominant alleles.
An example of this concept is seen with traits such as cleft chin. If the gene for a cleft chin (B) is dominant and the gene for a smooth chin (b) is recessive, a person with a heterozygous genotype (Bb) will display the cleft chin characteristic. When discussing the genetics of traits controlled by single genes, we often refer to Mendel's theory of inheritance, where the term heterozygous specifically denotes a genotype consisting of two different alleles.