Our bodies are intricate systems, and much of what makes us unique, from the color of our eyes to how tall we grow, is passed down through generations. This process, known as heredity, explains why offspring often resemble their parents, yet also exhibit individual differences. The instructions for these characteristics are encoded within our cells, forming the blueprint for life.
The Building Blocks: Genes and Traits
The fundamental units that determine these characteristics are called genes. A gene is a specific segment of DNA that holds the instructions for building and maintaining an organism. These instructions guide the production of proteins, which perform many functions within a cell and ultimately affect an organism’s features. Humans have 20,000 to 25,000 genes in each cell.
A trait is an observable characteristic or feature of an organism. Traits can be physical attributes, such as hair color or height, or even behavioral characteristics. Genes are directly responsible for these traits. For example, genes determine blood type and whether someone has freckles.
Variations in Genes and Traits
While genes provide the instructions for traits, variations exist within these instructions. These different versions of a single gene are called alleles. An individual inherits two alleles for each gene, receiving one from each biological parent.
Alleles can interact in different ways, commonly described as dominant or recessive. A dominant allele expresses its trait even if only one copy is present. For instance, the allele for brown eyes is dominant over the allele for blue eyes. If a person inherits one brown eye allele and one blue eye allele, their eyes will be brown.
In contrast, a recessive allele will only show its effect if an individual inherits two copies of it, one from each parent. Therefore, for someone to have blue eyes, they must inherit a blue eye allele from both parents.
Genetic Makeup Versus Observable Characteristics
Scientists use specific terminology to describe the relationship between genes and traits. An individual’s genetic makeup for a particular trait is called their genotype. This refers to the specific combination of alleles they possess for a given gene. For example, someone could have a genotype with two brown eye alleles, or one brown and one blue eye allele.
The observable physical or biochemical expression of that trait is known as the phenotype. While the genotype directly influences the phenotype, environmental factors can also play a role in how a trait is expressed.
An individual is considered homozygous for a trait if they have two identical alleles for that gene. This could mean having two dominant alleles or two recessive alleles. Conversely, an individual is heterozygous for a trait if they possess two different alleles for that gene, typically one dominant and one recessive. In a heterozygous individual, the dominant allele usually determines the observable phenotype.