DNA, the molecule containing the instructions that shape who we are. Genetics, the study of heredity, uses specific terms to describe these instructions and their variations. Delving into concepts like alleles and genotypes helps clarify the mechanisms behind our biological characteristics.
What is an Allele?
An allele represents a specific variant of a gene, which is a segment of DNA providing instructions for a particular trait or function. Genes are found at specific locations on chromosomes, and an individual typically inherits two copies of each gene, one from each parent. These different forms of a gene are what we refer to as alleles. For instance, a gene might determine eye color, but the alleles of that gene could specify brown, blue, or green eye color.
Alleles can exhibit different relationships, most commonly dominant or recessive. A dominant allele expresses its trait even when only one copy is present, effectively masking the effect of a recessive allele. For example, the allele for brown eyes is dominant over the allele for blue eyes; therefore, a person with one brown-eye allele and one blue-eye allele will have brown eyes. In contrast, a recessive allele only expresses its trait if an individual inherits two copies of it, one from each parent.
What is a Genotype?
A genotype refers to the specific combination of alleles an individual possesses for a particular gene. It is the unique genetic makeup inherited from parents for a given trait. For instance, if we consider the eye color gene, a person’s genotype might be represented as ‘BB’ for two brown-eye alleles, ‘Bb’ for one brown and one blue-eye allele, or ‘bb’ for two blue-eye alleles. This combination of alleles dictates the underlying genetic information.
Genotypes are categorized based on whether the two inherited alleles for a trait are identical or different. If an individual has two identical alleles for a gene, their genotype is described as homozygous (e.g., ‘BB’ or ‘bb’). Conversely, if the two alleles are different, the genotype is considered heterozygous (e.g., ‘Bb’).
How Alleles and Genotypes Work Together
Alleles and genotypes are linked, with alleles serving as the individual components that form a genotype. While an allele is a single variant form of a gene, the genotype is the specific pairing or collection of these alleles within an individual’s DNA for a particular characteristic. Each individual inherits one allele from each parent for most genes, and the combination of these two alleles constitutes their genotype for that specific trait.
The genotype determines the potential for an observable characteristic, known as the phenotype, and how the inherited alleles interact influences this expression. For example, in pea plants, a genotype with two purple-flower alleles (PP) or one purple and one white-flower allele (Pp) will both result in purple flowers because the purple allele is dominant. Only a genotype with two white-flower alleles (pp) will produce white flowers. This illustrates how different allele combinations within the genotype lead to distinct observable outcomes.