Genes are fundamental units of heredity, acting as the biological blueprint within our cells. A gene is a segment of DNA that carries specific instructions for creating proteins or performing particular functions. These genetic instructions dictate how we are built and function, influencing physical characteristics and complex biological processes.
Understanding Alleles
Different forms of a gene are known as alleles. An allele represents a specific variant of a gene. For instance, everyone has a gene that influences eye color, but the specific allele inherited determines whether that color is blue, brown, or green. Alleles are located at the same specific position, or locus, on homologous chromosomes, which are inherited one from each parent. For most genes, an individual inherits two alleles, one from each biological parent.
How Alleles Shape Traits
The interaction between these inherited alleles dictates an individual’s observable traits. Alleles can be classified as either dominant or recessive, describing how their associated traits are expressed. A dominant allele will express its trait even if only one copy is present, effectively masking the presence of a recessive allele. Conversely, a recessive allele will only manifest its trait if an individual inherits two copies of it, one from each parent.
The combination of alleles an individual possesses for a particular gene is called their genotype. This genetic makeup, in turn, influences the phenotype, which refers to the observable physical or biochemical characteristics resulting from that genotype. When an individual inherits two identical alleles for a specific gene (e.g., two dominant or two recessive alleles), they are considered homozygous for that trait. If they inherit two different alleles for the gene (one dominant and one recessive), they are heterozygous. In heterozygous individuals, the dominant allele typically determines the expressed phenotype.
The Origin of Allelic Variation
New alleles primarily come into existence through a process called mutation. A mutation is a change in the DNA sequence of a gene. These alterations can range from a single base pair change to larger insertions or deletions within the DNA. Mutations can occur spontaneously due to errors during DNA replication or be induced by environmental factors.
While some mutations can lead to harmful effects, many are neutral or even beneficial, providing new variations that may offer an advantage. Mutations are the ultimate source of genetic variation within a population, contributing to the diversity observed in living organisms.
Alleles and Human Diversity
The diverse array of human characteristics is largely a result of the many different alleles present within the human population. A common example is the ABO blood group system, which is determined by a single gene with three main alleles: A, B, and O. Alleles A and B are codominant, meaning both are expressed if inherited together, while the O allele is recessive. This allelic combination leads to four distinct blood types: A, B, AB, and O.
Eye color also illustrates allelic influence, though it is more complex, involving multiple genes. Brown eyes are generally considered dominant over blue eyes. Another example is earwax type, which can be wet or dry, determined by different alleles of a gene. These variations contribute to the unique genetic makeup and observable traits of each person.