Our physical characteristics, from the color of our eyes to the texture of our hair, are shaped by the intricate blueprint contained within our DNA. This blueprint, found in our DNA, holds the instructions that guide the development and function of every cell. Genetic inheritance determines the unique combination of traits that make each individual distinct.
Understanding Genes and Alleles
The fundamental units of this genetic blueprint are called genes, which are specific segments of DNA located along chromosomes. Each gene carries the instructions for a particular characteristic or function, such as producing a specific protein or influencing a trait like height. For instance, one gene might provide instructions for eye pigment, while another dictates blood type.
We inherit two copies of each gene, one from each biological parent. While a gene defines a particular trait, different versions of that gene exist, known as alleles. An allele is an alternative form of a gene, leading to different expressions of the same characteristic. For example, a gene for eye color might have an allele for brown eyes and another for blue eyes.
Heterozygous and Homozygous Genetic Makeup
An individual’s genetic makeup for a specific gene can be described as either heterozygous or homozygous. An individual is heterozygous for a gene if they have two different alleles for it. For example, if a person inherits an allele for brown eyes from one parent and an allele for blue eyes from the other, they are heterozygous for the eye color gene.
Conversely, an individual is homozygous for a gene if they possess two identical alleles for that gene. This occurs when they inherit the same variant from both parents. Using the eye color example, an individual would be homozygous if they received two brown eye alleles, or two blue eye alleles, one from each parent.
How Alleles Determine Observable Traits
The combination of alleles an individual possesses, their genotype, ultimately influences their observable characteristics, known as their phenotype. This expression often depends on the relationship between the two alleles present for a given gene. In many cases, one allele is dominant and can mask the presence of another allele, which is considered recessive.
A dominant allele expresses its associated trait even when only one copy is present in a heterozygous individual. For example, the allele for brown eyes is dominant over the allele for blue eyes. A person who is heterozygous for eye color (one brown allele, one blue allele) will have brown eyes because the brown allele overrides the blue allele.
A recessive allele only expresses its trait when two copies are present, meaning the individual must be homozygous for that recessive allele. If a person inherits two blue eye alleles, one from each parent, they will have blue eyes. While these principles apply to many traits, some characteristics are determined by the complex interplay of multiple genes and environmental factors.