When an individual is heterozygous, the 2 alleles can interact in a number of ways to affect the phenotype. Pedigrees are useful for looking at human traits. Controlled matings are powerful for looking at traits in other organisms. If a trait is Y-linked, only males will show it or pass it. If a trait is dominant, at least one of the parents of an affected person must be affected.
Most genes have more than one allele, but any individual willhave no more than two.
Incomplete dominance occurs when the phenotype is intermediate between the two extremes.