A gene is the fundamental unit of heredity, a specific segment of DNA that holds the instructions for building a particular protein, which determines a specific trait, such as eye color or blood type. While the location of a gene on a chromosome, known as its locus, is fixed, the DNA sequence within that location can vary slightly between individuals. These differences in the genetic code mean the instructions for a given trait are not always identical, allowing for the wide range of biological variation observed across a population.
Defining Alleles: The Forms of a Gene
The different versions of a specific gene are called alleles. Alleles are variations in the DNA sequence at the same physical spot on a chromosome and are responsible for the differences in the expression of a given trait, like having blue eyes versus brown eyes. For organisms that reproduce sexually, such as humans, two copies of every gene are inherited, one from each biological parent. This means that an individual possesses two alleles for every gene, which may be identical or different from each other.
If both inherited alleles are the same, the individual is said to be homozygous for that gene. Conversely, if the two inherited alleles are different, the individual is heterozygous.
Understanding Dominant and Recessive Alleles
The interaction between the two inherited alleles determines how the trait manifests, and the simplest form of this interaction is known as complete dominance. A dominant allele is one that only requires a single copy to be present for its associated trait to be expressed. The presence of this single dominant allele is enough to determine the physical trait.
A recessive allele, by contrast, will only result in the expression of its associated trait if two copies are present. In a heterozygous individual, the presence of the dominant allele masks or overrides the effect of the recessive allele, which is why the recessive trait is not visible. Geneticists often represent these interactions using a simple notation, where a capital letter represents the dominant allele and a corresponding lowercase letter represents the recessive allele.
Genotype, Phenotype, and Inheritance
The set of alleles an individual possesses for a particular gene is referred to as their genotype. This internal, genetic makeup is distinct from the physical trait that is ultimately observed, which is called the phenotype. The genotype dictates the potential range of the phenotype, but environmental factors can also influence the final expressed trait. For example, a person may have the genotype for tallness, but their final height, the phenotype, can be affected by nutrition.
If an individual is homozygous for a dominant trait (two dominant alleles), their genotype and phenotype are straightforwardly expressed. If they are heterozygous (one dominant and one recessive allele), the dominant allele determines the phenotype, even though the recessive allele is still part of the genotype. Only when an individual is homozygous for the recessive allele (two recessive alleles) does the recessive trait appear in the phenotype.
Complex Patterns of Allele Expression
Not all allele interactions follow the simple dominant and recessive pattern, and some traits are governed by more complex rules of expression. One such variation is incomplete dominance, where the heterozygous phenotype is a blend or intermediate of the two parental phenotypes. For instance, a cross between a red-flowered plant and a white-flowered plant might result in offspring with pink flowers, representing a mixture of the two colors.
Another complex pattern is codominance, in which both alleles are simultaneously and fully expressed in the heterozygous individual. The human ABO blood group system provides a well-known example, where the A allele and the B allele are codominant, meaning a person inheriting both will have type AB blood, expressing both A and B antigens equally on their red blood cells. Furthermore, some traits are controlled by multiple alleles, where a gene has more than two possible forms within a population, although any single individual still only possesses two of those forms. The ABO blood group system is also an example of multiple alleles, as it involves three different alleles: I^A, I^B, and i.