The unique characteristics of individuals, from hair color to blood type, are rooted in the fundamental units of heredity. This underlying genetic code holds the blueprints for all living organisms, guiding their development and shaping their observable traits.
Introducing Alleles
Different versions of a gene for the same trait are known as alleles. A gene is a specific segment of DNA that carries the instructions for a particular characteristic, such as eye color or blood type. Each gene is located at a fixed position, called a locus, on a chromosome. For most traits, an individual inherits two copies of each gene, with one allele coming from each parent. These two alleles, found at the same locus on homologous chromosomes, can be identical or different.
How Alleles Shape Our Traits
The interaction between the two inherited alleles dictates how a specific trait manifests in an individual, leading to observable characteristics known as the phenotype. The underlying genetic composition, or the combination of alleles an individual possesses, is referred to as the genotype.
A common interaction involves dominant and recessive alleles. 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 will only express its associated trait if an individual inherits two copies of it, one from each parent. 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. Blue eyes only appear if an individual inherits two copies of the recessive blue eye allele.
Beyond Simple Dominance
While dominant and recessive interactions explain many traits, allele interactions can be more complex. In incomplete dominance, neither allele is completely dominant over the other, resulting in a blended or intermediate phenotype in individuals with two different alleles. For instance, when red-flowered snapdragons are crossed with white-flowered snapdragons, the offspring may produce pink flowers. Similarly, the offspring of parents with straight and curly hair may have wavy hair.
Another type of interaction is codominance, where both alleles are fully expressed simultaneously, with neither allele masking the other. A clear example in humans is the ABO blood group system. If an individual inherits both the A allele and the B allele, they will have AB blood type, expressing both A and B antigens on their red blood cells. This differs from incomplete dominance, as both traits are distinctly visible rather than blended.
Some traits are also influenced by multiple alleles, meaning there are more than two possible allele versions for a gene within a population. The ABO blood group system also exemplifies multiple alleles, as there are three main alleles (A, B, and O) circulating in the human population, even though any single individual only inherits two of them.
Allele Origins and Inheritance
New alleles originate primarily through mutation, which involves random changes in the DNA sequence of a gene. These changes can occur due to errors during DNA replication or exposure to environmental factors like certain chemicals or radiation. If a mutation occurs in germ line cells (sperm or egg cells), it can be passed on to offspring, introducing a new allele into the population’s gene pool.
Alleles are passed from one generation to the next through sexual reproduction. During the formation of reproductive cells, a process called meiosis ensures that each parent contributes only one allele for each gene to their offspring. When fertilization occurs, the egg and sperm, each carrying one set of alleles, combine to form a new individual with a pair of alleles for each gene. This mechanism of inheritance ensures the continuity of genetic information while also allowing for genetic variation within a population.