The genetic blueprint stored within DNA contains instructions for producing proteins that determine an organism’s characteristics, such as eye color or disease susceptibility. To understand how these traits are passed down and expressed, it is necessary to examine the fundamental units of heredity. The allele is a core concept in this process, representing the specific variations that make up the instructions for a given feature.
Defining Alleles and Their Location
An allele is defined as a variant form of a gene, which is the basic unit of heredity that codes for a specific functional product, such as a protein. While a gene dictates a trait, like flower color, the different alleles are what determine the possible expressions of that trait, such as purple or white flowers. These variant forms arise through small differences in the DNA sequence at a specific point on a chromosome.
The physical address where an allele resides on a chromosome is called the locus. Since most multicellular organisms inherit one set of chromosomes from each parent, they possess two copies of nearly every gene. These two copies are found at the same locus on a pair of homologous chromosomes. An individual thus carries a pair of alleles for any given gene, one contributed by each biological parent.
The Interaction: Dominant and Recessive Traits
The pairing of two alleles for a trait leads to a functional relationship that determines which characteristic is physically expressed. This relationship is often described in terms of dominance, where one allele can mask the effect of the other. A dominant allele is one whose corresponding trait is always expressed whenever the allele is present in the pair.
The effect of a recessive allele is hidden or completely masked when paired with a dominant allele. The trait associated with a recessive allele only appears if an individual inherits two copies of that specific variant. Geneticists use a simple notation where a capital letter denotes the dominant allele (e.g., ‘A’), and the corresponding lowercase letter denotes the recessive allele (e.g., ‘a’).
Illustrative Examples of Allele Expression
The rules of dominance and recessiveness demonstrate how a pair of alleles translates into an observable trait, known as the phenotype. Consider the classic Mendelian trait of flower color in pea plants, where the allele for purple color is dominant over the allele for white color. If a plant inherits the purple allele (P) from one parent and the white allele (p) from the other, the plant will have purple flowers because the dominant allele is expressed.
Another example is the human trait of having a cleft chin, which is influenced by a single gene. The allele for a cleft chin is dominant (C), while the allele for a smooth chin is recessive (c). An individual who inherits even one dominant ‘C’ allele will develop a cleft chin, meaning they could have the allele combinations ‘CC’ or ‘Cc’. Only an individual who inherits the ‘c’ allele from both parents, resulting in the ‘cc’ combination, will exhibit the smooth chin phenotype.
Understanding Homozygous and Heterozygous Combinations
The specific combination of the two alleles inherited for a trait is known as the genotype. When an individual inherits two identical alleles for a particular gene, the genotype is described as homozygous. This state can be homozygous dominant, where both alleles are the dominant form (e.g., ‘CC’ for a cleft chin), or homozygous recessive, where both alleles are the recessive form (e.g., ‘cc’ for a smooth chin).
In contrast, a heterozygous genotype occurs when an individual inherits two different alleles for the same gene (e.g., ‘Cc’). In this case, the dominant allele determines the phenotype, as its effect masks the presence of the recessive allele.