Our bodies are built from instructions encoded in our genetic material, organized into distinct units called genes. We inherit one copy of most genes from each biological parent, meaning we carry two copies of nearly every gene. These gene copies are known as alleles, and they can be identical or different from each other. The specific combination of these alleles ultimately influences our individual characteristics.
Defining Heterozygous and Homozygous
The terms heterozygous and homozygous describe the composition of these allele pairs for a particular gene. An individual is considered heterozygous when they possess two different alleles for a specific gene, one inherited from each parent. For instance, if a gene determines eye color, a heterozygous individual might have one allele for brown eyes and another for blue eyes.
In contrast, an individual is homozygous when they carry two identical alleles for a given gene, meaning both parents contributed the same version. Using the eye color analogy, a homozygous individual would have two alleles for brown eyes or two alleles for blue eyes.
How Heterozygous Genes Determine Traits
When an individual is heterozygous, the interaction between their two different alleles dictates the observable characteristic, or phenotype. In many cases, one allele is dominant, meaning its trait is expressed, while the other allele is recessive and its trait is masked. For example, brown eye color alleles are dominant over blue eye color alleles. A person with one brown eye allele and one blue eye allele will have brown eyes.
Genetic expression can also follow more complex patterns beyond simple dominance. In incomplete dominance, the heterozygous phenotype appears as a blend of the two homozygous traits. In snapdragon flowers, a cross between a red-flowered plant and a white-flowered plant produces pink-flowered offspring. Codominance occurs when both alleles in a heterozygous pair are expressed simultaneously and distinctly. The human ABO blood group system is an example; individuals with AB blood type express both the A and B antigens on their red blood cells.
Inheritance of Alleles
The specific combination of alleles an individual receives is determined during reproduction. Each parent contributes one allele for each gene to their offspring. This process can be visualized and predicted using a Punnett square, a diagram showing the possible genetic outcomes of a cross between two parents.
Consider a cross between two parents who are both heterozygous for a single trait, such as flower color, where purple (P) is dominant and white (p) is recessive. Each parent, being heterozygous (Pp), can produce two types of gametes: one carrying the dominant ‘P’ allele and one carrying the recessive ‘p’ allele. When these gametes combine, the Punnett square reveals that approximately 50% of the offspring are expected to be heterozygous (Pp), 25% homozygous dominant (PP), and 25% homozygous recessive (pp). This helps predict the likelihood of offspring inheriting specific genotypes.
The Role of Heterozygosity in Health and Disease
Being heterozygous has implications for health, particularly concerning recessive genetic disorders. An individual who is heterozygous for a recessive disease allele is often referred to as a “carrier.” They possess one copy of the altered gene that could cause the disorder but do not exhibit symptoms due to their normal, dominant allele. For instance, a carrier of cystic fibrosis has one allele for the condition and one normal allele, and can pass the recessive allele to their children.
Beyond being carriers, heterozygosity can also provide a “heterozygous advantage,” where having two different alleles offers a protective benefit against certain environmental challenges. The sickle cell trait is an example. Individuals heterozygous for the sickle cell allele do not develop the severe form of sickle cell anemia. Instead, they exhibit increased resistance to malaria, a parasitic disease prevalent in some tropical regions. This protective effect arises because some sickled red blood cells make it more difficult for the malaria parasite to complete its life cycle, enhancing survival in malaria-prone environments.