Inheritance shapes every living organism, determining a vast array of characteristics from physical appearance to biological functions. At the heart of this process are fundamental units of heredity that carry life’s instructions. These components dictate how traits pass from one generation to the next, forming an individual’s blueprint.
Understanding Genes
Genes are fundamental units of heredity, specific segments of DNA that contain coded instructions. These instructions direct cells to produce proteins or functional RNA molecules, essential for building and maintaining an organism. Proteins perform various tasks within the body, from forming cellular structures to catalyzing metabolic reactions.
Each gene occupies a precise location on a chromosome, known as a locus. Humans possess an estimated 20,000 to 25,000 protein-coding genes. These genes vary in size, from a few hundred to over two million DNA base pairs. The sequence of these DNA bases (adenine, thymine, guanine, and cytosine) within a gene determines its specific instructions.
Understanding Alleles
An allele represents a specific variant form of a gene. While a gene provides instructions for a particular trait, alleles are the different versions of those instructions. For instance, a gene might determine eye color, but different alleles could specify blue, brown, or green eyes.
Multiple alleles can exist for a single gene within a population, leading to genetic diversity. An individual inherits two alleles for each gene—one from each biological parent. These alleles are located at the same position (locus) on homologous chromosomes, which are the paired chromosomes inherited from each parent.
Genes and Alleles Working Together
The difference between a gene and an allele lies in their scope: a gene is the general instruction for a characteristic, while an allele is a specific version of that instruction. Think of a gene as a recipe for a particular dish, such as cookies. Different alleles are variations of that recipe, like a chocolate chip cookie recipe versus an oatmeal cookie recipe. Both specify cookies, but the outcome is distinct based on the specific variant.
An individual inherits two alleles for each gene, receiving one from their mother and one from their father. The combination of these two alleles, known as the genotype, determines the observable trait, or phenotype. For example, the gene for eye color has various alleles, and the specific pair an individual inherits will dictate their eye color.
Alleles interact in different ways to produce traits. A dominant allele expresses its associated trait even if only one copy is present. A recessive allele, conversely, only expresses its trait if an individual inherits two copies of it, one from each parent. The human ABO blood group system provides an example of multiple alleles with different dominance patterns, where A and B alleles are codominant, and both are dominant over the O allele.