Heredity, the transmission of traits from one generation to the next, is encoded in an organism’s DNA. DNA stores the instructions for building and operating a body, governing characteristics like eye color or flower shape. To understand how variations arise and are passed on, one must understand the concept of an allele. An allele is a specific version or variant of a gene, and the combination of these variants determines an individual’s unique characteristics.
Defining Alleles and Genes
A gene is a functional segment of DNA that provides instructions for a cell to produce a specific protein or functional RNA molecule. Each gene governs a particular characteristic, such as the production of pigment. The fixed position on a chromosome where a gene is located is called its locus. Since humans and many organisms are diploid, they possess two copies of each chromosome, resulting in two loci for every gene.
An allele is the variation in the DNA sequence at that specific gene locus. For instance, a gene for flower color might have alleles that produce red pigment or white pigment. An individual inherits one allele from each biological parent, resulting in a pair of alleles for every gene. This pair dictates the specific expression of the trait.
The Mechanics of Allele Pairs
Allele pairs interact to determine the resulting trait through established patterns of expression. A dominant allele is one whose effect masks the effect of the other allele in the pair. The trait associated with a dominant allele is expressed even if only one copy is present. Conversely, a recessive allele is only expressed when an individual inherits two copies, one from each parent.
The terms homozygous and heterozygous describe the composition of an allele pair. An individual is homozygous if they have two identical alleles for a gene, such as two alleles for brown eyes. They are considered heterozygous if they have two different alleles, such as one brown and one blue eye allele. In a heterozygous pairing, the dominant allele determines the observable characteristic, while the recessive allele remains unexpressed.
Alleles and Observable Characteristics
The relationship between genotype and phenotype defines the distinction between the inherited code and the physical outcome. An organism’s genotype refers to the specific combination of alleles it possesses for a trait, representing the underlying genetic makeup. For instance, a person might have the genotype of one brown-eye allele and one blue-eye allele.
The phenotype is the resulting observable characteristic, which is the physical manifestation of the genotype. This is what can be seen or measured, such as brown eyes or a specific height. The rules of dominance and recessiveness dictate how the two alleles in the genotype interact to produce the phenotype. For example, the heterozygous genotype of one brown and one blue allele results in the brown-eye phenotype because the brown allele is dominant.
Allele Systems More Complex
While the simple dominant/recessive model explains many traits, allele expression is often more nuanced. Codominance is one pattern where both alleles in a heterozygous pair are fully and equally expressed, rather than one masking the other. The human ABO blood group system is a clear example; an individual with A and B alleles expresses both A and B surface markers, resulting in type AB blood.
Another variation is incomplete dominance, where the resulting phenotype is a blend or intermediate of the two parental phenotypes. For example, if a flower with alleles for red and white color exhibits incomplete dominance, the resulting flower will be pink. A gene can also have multiple alleles, meaning more than two variations of that gene exist within the population. The ABO blood group system also demonstrates this, as three alleles (A, B, and O) circulate in the human population, though any single person still only inherits a pair.