What Is a Genotype?

A genotype is the specific combination of alleles—variants of a gene—that a person carries at a particular location on the genome. In humans, each cell contains two copies of every chromosome, so for each gene we inherit one allele from our mother and one from our father.

Homozygous vs. Heterozygous

When the two alleles at a locus are identical, the individual is homozygous for that gene. If the alleles differ, the individual is heterozygous.

Using the classic notation where a capital letter denotes a dominant allele and a lowercase letter a recessive allele, the possible genotypes are:

Inheritance Patterns

Alleles interact to produce observable traits (phenotypes) through several inheritance patterns:

• Complete dominance: The dominant allele masks the recessive one. Both AA and Aa express the dominant phenotype; only aa shows the recessive trait.
• Incomplete dominance: The heterozygote shows an intermediate phenotype. Classic example: flower color in snapdragons (red × white = pink).
• Codominance: Both alleles are fully expressed. ABO blood type (IA × IB = AB) and blood cell antigen MN system illustrate this.

Real‑World Examples

• Hair color: Black (AA) vs. brown (Aa) vs. blond (aa).
• Blood type: A (IAIA or IAi), B (IBIB or IBi), AB (IAIB), O (ii).
• Sickle cell disease: Normal hemoglobin (HbA) is dominant over sickle hemoglobin (HbS). Heterozygotes (HbA/HbS) are carriers without symptoms.
• Huntington’s disease: One mutant allele (HD) is sufficient to manifest the disorder (dominant).

Why It Matters

Understanding whether a gene is homozygous or heterozygous informs risk assessment, carrier screening, and personalized medicine. Genetic counseling often uses this information to predict the likelihood of passing traits or disorders to offspring.

By mastering these basics, you gain a clearer view of how your DNA shapes you and your family's health.