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For a specified characteristic, complete dominance means that the heterozygote has the same phenotype as one of the two homozygotes. It does not mean that the dominant allele is more common, more advantageous, or transmitted more often. Penetrance specifies the conditional probability of an expressed phenotype given a genotype and is a separate modeling assumption.

A genotype records the inherited alleles carried by an individual at one or more genetic loci. At a two-copy diploid genetic locus with alleles $A,a$, the possible unordered genotypes are $AA,Aa,aa$. The first and last are homozygotes; the middle is a heterozygote. The notation records inherited states, without yet asserting their phenotypes. See NHGRI: genotype.

An unordered $Aa$ combines the two parental orders $Aa$ and $aA$. This small bookkeeping point explains the factor two in Hardy-Weinberg proportions and in an $Aa\times Aa$ cross. If parental origin or arrangement across several genetic loci matters, a phase-known genotype retains extra information about the two haplotypes.

A heterozygote has greater genotype fitness than either corresponding homozygote. In the deterministic random-mating two-allele viability-selection model this gives an attracting interior allele frequency, unlike the repelling interior frequency produced by underdominance.

Homozygosity means that the two allele states at a diploid genetic locus are the same; an individual with that property is a homozygote. Observed homozygosity does not by itself establish identity by descent.

A test cross crosses an individual with a homozygote for recessive alleles, allowing suitable offspring phenotypes to reveal transmitted gamete combinations.

Underdominance means that the heterozygote has lower genotype fitness than both corresponding homozygotes. In a deterministic randomly mating biallelic population, it produces an unstable interior allele-frequency threshold separating the two fixation outcomes, unlike heterozygote advantage.

With homozygote fitness advantages $a,b>0$ over the heterozygote, the allele frequency $u$ of the first allele has an unstable threshold $b/(a+b)$. Below it that allele decreases; above it it increases. In a discrete viability-selection generation the same numerator gives $u^{\prime }-u$ after division by mean genotype fitness. Equal homozygote advantages give $\dot{u}=su(1-u)(2u-1)$, with stable endpoints and unstable midpoint.