Heterozygous

In genetics, the term 'heterozygous' describes an individual or genotype possessing two different alleles for a particular gene. Alleles are alternative forms of a gene that occupy corresponding positions (loci) on homologous chromosomes. The heterozygous genotype contrasts with a homozygous genotype, which has two identical alleles for the same gene. The resulting phenotype (observable characteristics) of a heterozygous individual is often influenced by the interaction between the two alleles; one allele might be dominant, masking the effect of the other (recessive) allele. Other interactions between alleles include codominance and incomplete dominance, resulting in different phenotypes. This state signifies genetic diversity within an individual and contributes to the broader genetic variation within a population. It is a core concept in understanding inheritance patterns and predicting offspring phenotypes.

Heterozygous meaning with examples

• A person carrying one allele for brown eyes (B) and one allele for blue eyes (b) would be heterozygous (Bb). Since brown eyes are dominant, the individual's phenotype would be brown eyes. This is a classic example of how heterozygous genotypes result in an observable trait, even when a recessive allele is present. This shows how an individual can carry a trait without expressing it due to the dominance principle.
• In a population of snapdragons, the allele for red flowers (R) is incompletely dominant over the allele for white flowers (r). A heterozygous (Rr) snapdragon would exhibit a pink flower phenotype, an intermediate expression of the two alleles. This demonstrates that the phenotype is a blend of the parental traits. Here, the heterozygous genotype manifests a novel phenotype due to the incomplete dominance.
• Consider a disease where one allele (D) confers the disease, while the other (d) is normal. A person with the heterozygous (Dd) genotype might show a milder form of the disease or be a 'carrier' who doesn't express any symptoms. This example emphasizes how the heterozygous state can sometimes lead to asymptomatic carriers, highlighting the impact of genetic diversity.
• A farmer breeding cattle observes that the gene for hornlessness (H) is dominant to the gene for horns (h). If he crosses a heterozygous (Hh) bull with a homozygous recessive (hh) cow, there is a 50% chance that the offspring will be hornless and a 50% chance that the offspring will have horns. The differing allele combinations in the offspring lead to varying phenotypes.