DNA contains the instructions for our bodies, organized into units of heredity called genes. Humans inherit characteristics such as hair color, blood type, and height from their biological parents through these genes. An individual’s genotype is the specific combination of genetic information they possess, which dictates their observable traits, or phenotype. Understanding the term “heterozygous” is fundamental to grasping how this genetic inheritance system works, as it describes a common condition within our biological makeup.
Understanding the Building Blocks: Genes and Alleles
A gene is a distinct segment of DNA located at a specific position on a chromosome that codes for a particular trait or protein. Since chromosomes come in pairs, an individual inherits two copies of each gene—one from each biological parent. These inherited genes determine the potential for a specific characteristic, such as eye color or blood type.
The specific variations of a gene are known as alleles. For example, the gene for eye color might have an allele for brown pigment and a different allele for blue pigment. These alleles occupy the same location, or locus, on the pair of chromosomes. The combination of these two alleles constitutes an individual’s genotype for that specific trait.
The Difference Between Heterozygous and Homozygous
The term “heterozygous” literally means having different components, which in genetics refers to having two dissimilar alleles for a particular gene. An individual with a heterozygous genotype has inherited one version of a gene from one parent and a different version from the other parent. This state is typically represented in genetic notation using one uppercase letter and one lowercase letter, such as “Aa,” where the letters represent the two different alleles.
In contrast, the term “homozygous” describes a genotype where the two inherited alleles for a gene are identical. This can occur in two forms: homozygous dominant (AA) or homozygous recessive (aa).
Homozygous dominant means the individual possesses two copies of the allele whose trait is expressed more strongly. Conversely, homozygous recessive means they possess two copies of the allele whose trait is expressed less strongly. The heterozygous state (Aa) is structurally distinct from both homozygous conditions.
How Heterozygous Status Affects Physical Traits
The relationship between the two different alleles in a heterozygous genotype determines the resulting observable trait, or phenotype. In cases of complete dominance, the trait associated with the dominant allele completely masks the effect of the recessive allele. For instance, if ‘A’ is the dominant allele for dark hair pigment and ‘a’ is the recessive allele for lighter pigment, a person with the ‘Aa’ genotype will have dark hair.
The recessive allele is still present in the heterozygous genotype, but its trait is not expressed externally. This pattern is known as Mendelian inheritance, where the dominant allele requires only one copy to be fully expressed. However, not all traits follow this simple dominant-recessive pattern when an individual is heterozygous.
Incomplete Dominance
Other inheritance patterns, such as incomplete dominance, result in a blended phenotype in the heterozygous state. For example, if a flower has one allele for red color and one for white, the heterozygous flower may appear pink.
Codominance
Codominance is an exception where both alleles are fully and distinctly expressed at the same time. This is seen in the human AB blood type, where both A and B antigens are produced from the heterozygous ‘AB’ genotype.
The Role of Heterozygosity in Genetic Inheritance
The heterozygous state plays a significant part in the transmission of traits and genetic diversity across generations. Because a heterozygous individual possesses two different alleles, they have a 50% chance of passing on either one of those alleles to their offspring. This mechanism is a primary source of genetic variation within a family line and a population.
An individual who is heterozygous for a gene with a recessive trait or disorder is often referred to as a “carrier.” The dominant, typically healthy, allele ensures the individual does not display the recessive condition. However, the recessive allele can still be passed to the next generation, potentially leading to the disorder if the offspring inherits the same recessive allele from the other parent.
If two parents are both heterozygous for a single recessive trait (Aa x Aa), their children face specific probabilities for inheriting the alleles:
- 25% chance of inheriting two dominant alleles (AA).
- 50% chance of inheriting one of each allele (Aa).
- 25% chance of inheriting two recessive alleles (aa).
The heterozygous state is therefore a key mechanism for maintaining recessive alleles in the gene pool, allowing them to remain hidden for generations until the specific combination occurs.