A gene is a functional segment of deoxyribonucleic acid (DNA) that carries instructions for building a specific protein or functional ribonucleic acid (RNA) molecule. These instructions contribute to a particular characteristic, or trait, in an organism. Every gene is situated at a specific physical location, known as a locus, on a chromosome.
Alleles are the different versions of a single gene. For instance, a gene might determine hair texture, while its alleles determine if the texture is curly, wavy, or straight. Since humans inherit one set of chromosomes from each parent, every individual possesses two alleles for each gene, which determine the final observable trait.
Understanding Allele Function
The interaction between the two inherited alleles determines how a trait is physically expressed. In the simplest form of inheritance, one allele is described as dominant and the other as recessive. The dominant allele expresses its characteristic even when only one copy is present, masking the presence of the recessive allele.
This relationship is based on the protein product created by the allele. A dominant allele codes for a fully functional protein, and a single copy is sufficient to produce the associated trait. Conversely, a recessive allele often contains a variation resulting in a non-functional or less functional protein.
An individual carrying two identical alleles is described as homozygous for that gene. If an individual inherits two different alleles—one dominant and one recessive—they are called heterozygous, though the dominant trait will be observed. The recessive trait is only expressed when a person inherits two copies of the recessive allele, meaning they are homozygous recessive.
Common Allele Examples in Human Traits
Several common human physical features illustrate the dominant and recessive pattern. One example is the attachment of earlobes, often cited as a simple Mendelian trait. The allele for unattached earlobes is dominant over the allele for attached earlobes.
Another observed trait is the hairline shape, specifically the presence of a widow’s peak. The allele that results in a distinct, V-shaped point is dominant. A straight hairline results from inheriting two copies of the recessive allele.
Facial dimples and a cleft chin also follow this basic inheritance pattern. The allele for cheek dimples is dominant over the allele for no dimples. Similarly, the allele that produces a cleft chin is dominant over the allele for a smooth chin. These traits demonstrate how a single dominant allele can determine a visible characteristic.
Alleles Showing Complex Relationships
While many traits follow the simple dominant-recessive model, other characteristics are governed by more complex allelic relationships. One scenario is multiple alleles, where a gene has more than two possible variations within the population. The human ABO blood type system is the most recognized example, involving three common alleles: I-A, I-B, and i.
The I-A and I-B alleles code for specific carbohydrate markers, or antigens, on the surface of red blood cells, while the i allele does not code for any marker. Both I-A and I-B are dominant over the i allele; thus, an individual with genotype I-A i has Type A blood, and I-B i has Type B blood. The i i genotype results in Type O blood.
The system also demonstrates codominance, where both alleles are fully expressed simultaneously. If a person inherits both the I-A and I-B alleles, the resulting blood type is AB, meaning both A and B antigens are present on the red blood cells.