Heredity describes the process by which traits are passed from parents to their offspring. This explains why individuals resemble their family members but also exhibit unique characteristics. The genetic information within an organism dictates its physical attributes and even some behavioral tendencies. Understanding how this information is organized and expressed provides insight into the diverse forms of life.
The Basics of Alleles
Genetic information is stored in long molecules of DNA, organized into structures called chromosomes. A gene is a specific segment of this DNA that provides instructions for building a particular protein or performing a specific function. For most genes, individuals inherit two copies, one from each biological parent.
These different versions of a gene are called alleles. Alleles occupy the same specific position, known as a locus, on homologous chromosomes. While they code for the same trait, such as eye color, different alleles can lead to variations in that trait, like blue eyes versus brown eyes. The combination of these alleles determines an individual’s genetic makeup, or genotype.
Understanding Dominant Alleles
A dominant allele is a variant of a gene that expresses its associated trait even when only one copy is present. The effect of any other allele present for that same gene will be masked by the dominant one. For instance, the allele for brown eyes is dominant over the allele for blue eyes. Therefore, if a person inherits one allele for brown eyes and one for blue eyes, their eyes will be brown. Dominant alleles are often represented by a capital letter, such as ‘B’ for brown eyes.
Understanding Recessive Alleles
A recessive allele expresses its associated trait only when two copies are present; if a dominant allele is also present, its effect is hidden and not observed. Using the eye color example, the allele for blue eyes is recessive. For a person to have blue eyes, they must inherit a blue-eye allele from both their mother and their father. If they inherit even one brown-eye allele, their eyes will be brown because the dominant brown-eye allele masks the recessive blue-eye allele. Recessive alleles are typically represented by a lowercase letter, such as ‘b’ for blue eyes.
How Allele Combinations Determine Traits
The interaction between dominant and recessive alleles dictates how traits are expressed in an organism. Each individual inherits two alleles for most genes, one from each parent, forming their genotype. The observable characteristic that results from this genotype is called the phenotype.
When an individual inherits two identical alleles for a particular gene, they are said to be homozygous for that trait. If both alleles are dominant (e.g., ‘BB’), the individual is homozygous dominant, and the dominant trait will be expressed. If both alleles are recessive (e.g., ‘bb’), the individual is homozygous recessive, and the recessive trait will be expressed.
A heterozygous individual inherits two different alleles for a gene, one dominant and one recessive (e.g., ‘Bb’). In this case, the dominant allele will override the recessive allele, and the dominant trait will be expressed. For example, someone with one allele for detached earlobes (dominant) and one for attached earlobes (recessive) will have detached earlobes. This interplay between dominant and recessive alleles explains the diversity of traits observed across populations.