| Criteria | Homozygous | Heterozygous | Remarks |
| Definition | Condition in which an individual carries two identical alleles of a particular gene (e.g., AA or aa). | Condition in which an individual carries two different alleles of a particular gene (e.g., Aa). | Represents different zygosity states at a gene locus. |
| Genetic Composition | Both alleles on homologous chromosomes are the same. | Alleles on homologous chromosomes are different. | Determines how traits are expressed. |
| Expression of Traits | Always expresses the same trait, whether dominant or recessive. | Expression depends on the dominance relationship (dominant allele masks recessive). | Heterozygosity often results in hybrid or variable traits. |
| Phenotypic Effect | Recessive homozygotes show recessive traits; dominant homozygotes show dominant traits. | Typically shows the dominant trait; may show intermediate phenotype in incomplete dominance or codominance. | Expression patterns vary with genetic context. |
| Genetic Variation | Does not contribute to variation at that locus. | Contributes to genetic diversity and variation within a population. | Heterozygosity is important for evolution and adaptability. |
| Examples | Cystic fibrosis gene: aa (homozygous recessive) leads to disease; AA (homozygous dominant) normal. | Sickle-cell gene: AS (heterozygous) provides malaria resistance but not full disease. | Shows importance in health and disease. |
