Dominantrecessive

Dominant and recessive are terms used to describe how certain alleles influence the phenotype of an organism. For a given gene, one allele can be dominant and the other recessive. In a simple dominant–recessive model, the dominant allele masks the effect of the recessive allele in a heterozygous individual. Therefore, individuals with two copies of the dominant allele (AA) or one copy of each allele (Aa) typically show the dominant phenotype, while only individuals with two recessive alleles (aa) show the recessive phenotype.

Genotype and phenotype: Each person has two copies of most genes. The combination of these alleles determines the observable trait. A heterozygote (Aa) usually displays the dominant trait, while a homozygous recessive (aa) displays the recessive trait. Carriers of a recessive allele (for example, Aa) may not show the recessive phenotype but can pass the allele to offspring.

Common patterns: In autosomal dominant conditions, one copy of the dominant allele is sufficient to affect the trait or cause disease (e.g., Huntington’s disease). In autosomal recessive conditions, two recessive alleles are required (e.g., cystic fibrosis). X-linked recessive inheritance can more often affect males, who have a single X chromosome.

Limitations: Not all genes follow simple dominance. Some exhibit incomplete dominance, where the heterozygote has an intermediate phenotype, or codominance, where both alleles contribute to the phenotype. Environmental factors can also influence expression.