Heredity is the process by which characteristics are passed from parents to their offspring. At the core of this process is deoxyribonucleic acid, or DNA, which serves as the blueprint of life. Genes, segments of DNA, contain the coded instructions that dictate various features and functions within an organism.
Understanding Alleles
Alleles are different versions of a specific gene. Imagine a gene as a recipe for a characteristic like eye color; alleles would be the different versions of that recipe, such as for brown eyes or blue eyes. These variations arise from slight differences in the DNA sequence at a particular location on a chromosome. Humans inherit two alleles for each gene, with one coming from each biological parent. These two alleles are located at the same position, or locus, on homologous chromosomes.
Allele Combinations and Traits
The combination of the two alleles inherited for a gene determines an individual’s observable trait, known as the phenotype. The specific set of alleles an individual possesses for a gene is called their genotype. Alleles interact in various ways, most commonly through a dominant-recessive relationship. A dominant allele expresses its trait even when only one copy is present, effectively masking the presence of a recessive allele. Conversely, a recessive allele only expresses its trait if two copies are present.
When an individual inherits two identical alleles for a gene, their genotype is described as homozygous. This can be homozygous dominant (two dominant alleles) or homozygous recessive (two recessive alleles). If an individual inherits two different alleles for a gene, their genotype is described as heterozygous. In a heterozygous individual, the dominant allele’s trait will be expressed, while the recessive allele’s trait remains hidden.
Illustrative Allele Examples
Several human traits demonstrate how allele combinations lead to observable characteristics. Eye color, for instance, is influenced by multiple genes, but in a simplified model, brown eye color is dominant over blue eye color. An individual with one brown eye allele and one blue eye allele will typically have brown eyes. Blue eyes appear only when an individual inherits two copies of the recessive blue eye allele, one from each parent.
Earlobe attachment is another example, though its genetics are more complex than a simple dominant/recessive model. Historically, unattached earlobes were thought to be dominant and attached earlobes recessive, controlled by a single gene. However, research indicates that earlobe attachment is influenced by many genes, with at least 49 genes identified as contributing factors, and varying degrees of attachment exist. This shows that even seemingly simple traits can involve complex genetic interactions.
ABO blood types offer a more intricate example involving multiple alleles and different dominance patterns. The ABO gene has three common alleles: A, B, and O. Alleles A and B are codominant, meaning both are expressed if inherited together, resulting in AB blood type. Both A and B alleles are dominant over the O allele, which is recessive. This means an individual with an A allele and an O allele will have type A blood, and similarly for type B. Only individuals inheriting two O alleles will have type O blood.