Traits are observable characteristics of an organism, such as eye color, hair texture, or disease resistance. Our traits are largely determined by our genetic makeup, which is passed down from our parents. Environmental factors can also influence how these characteristics develop and appear.
The Building Blocks: Genes and Alleles
Genes serve as the fundamental units of heredity, transmitting information from parents to their offspring. Each gene contains instructions for building specific proteins, which then influence the development of various traits. Most genes exist in different versions, known as alleles. For example, a gene for eye color might have alleles for brown, blue, or green eyes.
Individuals inherit two alleles for each gene, receiving one from each parent. The specific combination of these alleles an individual possesses is called their genotype. This genetic blueprint then manifests as the observable trait, or phenotype. When an individual inherits two identical alleles for a particular gene, they are considered homozygous for that gene. Conversely, if they inherit two different alleles, they are heterozygous.
Understanding Dominant and Recessive Traits
Dominant and recessive describe how different alleles interact to produce a specific trait. A dominant trait is always expressed if at least one copy of the dominant allele is present, meaning it will be seen even in a heterozygous genotype.
In contrast, a recessive trait will only appear when an individual inherits two copies of the recessive allele. If a dominant allele is also present, its effect masks the recessive allele’s influence. For instance, brown eye color is often considered dominant over blue eye color. An individual with one allele for brown eyes and one for blue eyes will typically have brown eyes because the brown allele is dominant. Blue eyes only occur when both inherited alleles are for blue eyes.
How Traits Are Inherited and Expressed
Parents pass on one allele from each gene pair to their offspring. This combination determines the offspring’s unique genetic makeup, which then dictates the observable trait based on dominance and recessiveness. For example, if both parents are heterozygous for a trait, their offspring can inherit two dominant alleles, two recessive alleles, or one of each.
Consider a simplified example like attached versus unattached earlobes, where unattached earlobes are dominant. An individual with two alleles for unattached earlobes will have unattached earlobes, as will someone with one allele for unattached and one for attached earlobes. Only an individual with two alleles for attached earlobes will display that particular trait. People who carry a recessive allele but do not express the trait themselves are known as carriers. They possess the genetic information for the recessive trait and can pass it on to their children, even though it is not visible in their own phenotype.
Common Misconceptions About Dominance
A common misunderstanding is that dominant traits are always more prevalent in a population. However, “dominant” refers to how an allele is expressed when paired with another, not its frequency. For example, extra fingers or toes (polydactyly) is a dominant trait, yet it is relatively uncommon in the general population.
Another misconception is that dominant traits are inherently “better” or stronger. Dominance simply describes an allele’s observable expression, not its impact on fitness or health. Both dominant and recessive alleles can be associated with beneficial, neutral, or harmful conditions. For instance, some genetic disorders are caused by a single dominant allele, while others require two copies of a recessive allele. Recessive traits do not disappear from a population; recessive alleles persist through generations, carried by heterozygous individuals who do not express the trait.