Heredity and variation are governed by the instructions contained within deoxyribonucleic acid, or DNA. These molecular strands hold the blueprints that determine the unique traits of every living organism. Understanding the terms used to describe how these instructions vary is fundamental to grasping the mechanisms of biological inheritance.
The Building Blocks: Genes and Alleles
The basic unit of heredity is the gene, a specific segment of DNA that provides the code for a functional product, such as a protein. Each gene resides at a specific position on a chromosome, a location known as its locus. Organisms inherit one set of chromosomes from each parent, meaning they receive two copies of every gene.
These two copies of a gene may not be identical; the different versions are called alleles. An allele is a specific variant of a gene. For example, a gene for eye color may have one allele that codes for brown eyes and another that codes for blue eyes. The combination of these two alleles determines the genetic makeup for that trait.
Defining Homozygous and Heterozygous
The terms homozygous and heterozygous describe the specific pair of alleles an individual carries at a given gene locus. This specific allelic combination is known as the genotype. A person is considered homozygous for a trait if they have two identical alleles for that gene. This is represented by either two dominant alleles (“AA”) or two recessive alleles (“aa”).
Conversely, an individual is heterozygous when they possess two different alleles for a specific gene. This genotype is represented by one uppercase and one lowercase letter, such as “Aa.” The heterozygous state means the two inherited genetic instructions are distinct variants of the same gene.
Connecting Genotype to Physical Traits
The genotype determines the phenotype, which is the observable physical characteristic or trait resulting from the genetic makeup. In many cases of simple inheritance, one allele is dominant, meaning it masks the effect of the other allele. The allele that is masked is called recessive.
If an individual is homozygous dominant (AA), the resulting phenotype will express the dominant trait. A homozygous recessive individual (aa) will display the recessive trait because there is no dominant allele present to mask it. This is the only genotype where the recessive trait appears in the phenotype.
When an individual is heterozygous (Aa), they possess both a dominant and a recessive allele. The presence of the single dominant allele is usually enough to determine the observable trait. Therefore, a heterozygous (Aa) individual will display the same dominant phenotype as a homozygous dominant (AA) individual.