Dominant

A dominant allele is one that will determine the phenotype even if only one copy is present. In other words, a trait is called dominant if a single copy of the responsible allele is enough to make that trait appear (masking the effect of any recessive allele).

• Heterozygous (one dominant and one recessive allele) and homozygous dominant (two dominant alleles) individuals look the same phenotypically - the dominant trait is expressed in both cases.
• Dominant does not necessarily mean common. For example, having extra fingers (polydactyly) is caused by a dominant allele, yet most people have five digits (the recessive condition). Many dominant genetic disorders (like achondroplasia) are rare in the population.
• In autosomal dominant inheritance, an affected person usually has an affected parent (the trait typically appears in every generation). Each child of an affected parent has a 50% chance of inheriting the condition (assuming the parent is heterozygous).

• Pedigree clues for an autosomal dominant trait: it tends to appear in every generation. Unaffected parents do not produce affected children (no skipping of generations, barring new mutations).
• If one parent is affected with a single dominant allele, typically about half of the offspring will inherit the trait - a classic 50% pattern often tested in questions about probability.
• Be cautious with terminology: in dominant conditions there are no "carriers" without symptoms. If someone carries the dominant allele, they generally show the trait. Exams may ask you to distinguish this from recessive patterns, where carriers have the allele but not the trait.