- The term heterozygous describes the genetic condition in which an individual carries two different alleles at a given genetic locus on homologous chromosomes. Since most organisms, including humans, are diploid, they inherit one copy of each chromosome from their mother and one from their father. At any locus on these paired chromosomes, the DNA sequence can either be identical (homozygous) or different (heterozygous). When the alleles differ, the individual is said to be heterozygous at that locus. For example, at a locus influencing blood type in humans, one chromosome might carry an allele for type A, while the homologous chromosome carries an allele for type B, making the individual heterozygous for that trait.
- Heterozygosity has important consequences for genetics and inheritance. In many cases, heterozygosity results in a dominant-recessive relationship between alleles. If one allele is dominant, its trait will be expressed in the phenotype, while the recessive allele’s effect will remain hidden. For example, in Mendel’s classic pea experiments, plants heterozygous for flower color (one purple allele and one white allele) displayed purple flowers because the purple allele was dominant. However, not all heterozygous states follow this simple pattern. In incomplete dominance, the phenotype represents an intermediate between the two alleles, such as red and white flower alleles producing pink flowers. In codominance, both alleles are expressed equally, as in the human ABO blood group system, where an individual with both A and B alleles expresses both antigens on red blood cells.
- On a molecular and population level, heterozygosity is also a key measure of genetic variation. The proportion of heterozygous loci within an individual, or across a population, reflects genetic diversity, which is essential for adaptability and evolutionary potential. Populations with high heterozygosity generally have more resilience to environmental changes and disease outbreaks because the presence of diverse alleles increases the likelihood of adaptive traits. Conversely, low heterozygosity, often caused by inbreeding, can reduce genetic diversity and increase the prevalence of harmful recessive conditions, as individuals are more likely to inherit identical alleles.
- In medicine and genetics, identifying heterozygous individuals can have important implications. Carriers of genetic diseases often appear healthy because they are heterozygous for a disease-causing allele (recessive) and a normal allele. For example, individuals who are heterozygous for the sickle cell allele (HbA/HbS) typically do not develop severe sickle cell anemia, yet they gain some protection against malaria. This illustrates how heterozygosity can have both protective and hidden effects on human health. Advances in genetic testing now allow for the detection of heterozygous states across the genome, aiding in disease risk assessment, carrier screening, and personalized medicine.
