Final answer:
A dominant allele is represented by a capital letter and only one copy is required for the dominant trait to be expressed. An individual's genotype can be homozygous or heterozygous, comprising dominant and recessive alleles. While Mendel's work focused on two allele types per gene, modern genetics acknowledges multiple alleles within a population.
Step-by-step explanation:
A single copy of a gene is enough to express its dominant trait, typically denoted by a capital letter. In genetics, these are referred to as dominant alleles. For example, if 'B' represents the dominant allele for a cleft chin, then an individual with either 'BB' or 'Bb' genotypes will express the cleft chin phenotype since it only requires one copy of the 'B' allele.
Alleles are variants of a gene located at the same locus on homologous chromosomes. While Gregor Mendel's work suggested that there are only two types of alleles for each gene—one dominant and one recessive—modern genetics has revealed a broader spectrum of allele interactions, including multiple alleles within a population. However, an individual can only have two of these alleles. Conventions in genetic nomenclature use uppercase letters for dominant alleles and lowercase letters for recessive alleles. Therefore, a homozygous dominant genotype would be denoted as 'BB', a homozygous recessive genotype as 'bb', and a heterozygous individual carrying one of each as 'Bb'.
It is also important to note that while some traits are determined by a single gene, many are controlled by multiple genes, such as human eye color, which is influenced by eight or more genes. Additionally, in cases like blood type, there can be multiple alleles present in a population, like 'IA', 'IB', and 'i' for the ABO blood system.