The question of whether an allele is represented by one letter or two is a common source of confusion when first learning about genetics. Heredity focuses on how characteristics are passed from one generation to the next, governed by discrete units of inheritance known as genes. Genes provide the instructions for building and maintaining an organism, encoding traits from eye color to enzyme production. Understanding genetic notation requires recognizing the difference between a single variant instruction and the inherited pair of instructions an organism carries.
Defining the Allele
A gene is a specific sequence of DNA located on a chromosome that codes for a particular functional product, such as a protein. Alleles are the different versions or variants of that specific gene. For instance, one allele of a gene might specify purple flowers, while another specifies white flowers.
The physical address on a chromosome where a gene and its alleles are situated is known as the locus. Diploid organisms, including humans, inherit two complete sets of chromosomes, one from each parent. Consequently, an individual carries two alleles for every gene, one on each of the paired homologous chromosomes.
The Notation Confusion: One Letter vs. Two
The standard notation used in classical genetics clarifies the distinction between the single variant and the inherited pair. A single letter, such as ‘A’ or ‘a’, represents a single allele, which is the unit inherited from one parent. This letter symbolizes the specific version of the gene present on one chromosome.
The two-letter notation, such as ‘AA’, ‘Aa’, or ‘aa’, represents the genotype, which is the complete genetic makeup for that single trait. Since sexually reproducing organisms receive one allele from each parent, the genotype is always represented by the pairing of these two inherited letters. The offspring combines the two single alleles received from both parents to form its two-letter genotype.
Allele Interaction and the Genotype
The two-letter genotype identifies the inherited alleles and predicts the resulting physical characteristic, known as the phenotype. This is achieved by using different cases of the same letter; capital letters indicate dominant alleles, and lowercase letters indicate recessive alleles. A dominant allele is one whose effect is expressed in the phenotype even when only one copy is present.
A recessive allele, conversely, only determines the phenotype if two copies of it are inherited. For example, a genotype of ‘AA’ or ‘Aa’ would both display the trait associated with the dominant ‘A’ allele. The recessive trait is only physically expressed if the genotype is ‘aa’.
When both alleles in the genotype are identical, such as ‘AA’ or ‘aa’, the individual is said to be homozygous for that gene. If the two alleles are different, like ‘Aa’, the individual is heterozygous.