Sometimes both inherited copies of a gene are identical. Homozygous describes the condition of having two of the same allele at a particular locus. The alleles may both be dominant or both recessive.

Homozygous dominant individuals express the dominant phenotype; homozygous recessive individuals express the recessive phenotype. For recessive disorders, only the homozygous recessive genotype produces the disease. Inbreeding increases the chance of homozygosity for rare alleles.

Genetic tests report homozygous genotypes when only one allele is detected. Homozygosity mapping helps identify recessive disease genes in consanguineous families. The concept is essential for predicting Mendelian ratios and understanding the genetic composition of populations.

• Having two identical alleles at a locus
• Can be dominant or recessive
• Required for expression of recessive traits
• Increased by relatedness of parents