Final answer:
When an individual possesses two different alleles for a gene, one dominant and one recessive, they are considered heterozygous. The genotype would typically be written Aa, where the dominant allele A is expressed in the phenotype. However, some genetic expressions, such as codominance, may show both alleles in a heterozygous individual.
Step-by-step explanation:
When an individual has two different alleles for a particular gene, that individual is said to be heterozygous. This implies that one allele is dominant and the other is recessive, and they are located on the homologous chromosomes. Examples of how genotypes are expressed include heterozygous genotypes written as Aa, with the dominant allele (A) always written before the recessive allele (a). When only the dominant trait is expressed in the phenotype, this is a result of conventional Mendelian inheritance.
It's important to recognize that not all genetic traits follow the simple dominant-recessive relationship. Sometimes alleles exhibit codominance or incomplete dominance, leading to phenotypes that differ from traditional dominance patterns. However, when considering classical Mendelian genetics, the heterozygous genotype expresses the dominant trait, making it phenotypically indistinguishable from the homozygous dominant condition.