Beyond Simple Dominance

Objective: Distinguish intermediate from jointly expressed heterozygote phenotypes.

Complete dominance is not the only genotype-to-phenotype relationship. In incomplete dominance, a heterozygote has an intermediate measured phenotype in a specified context; in codominance, both allele-associated products or features can be detected in the heterozygote. Neither pattern means alleles blend irreversibly in DNA: meiosis can still segregate intact alleles. A cross between two heterozygotes may produce a 1:2:1 phenotype ratio if each genotype is distinguishable and viability is comparable. Whether an allele appears dominant, incomplete, or codominant can depend on the level of observation, such as protein versus organism trait.

Objective: Explain allele diversity and multifactorial traits without simplistic labels.

A population may contain more than two alleles at a locus, although a typical diploid individual usually carries at most two at once. Polygenic traits depend on variants at multiple loci; environmental factors and gene interactions can also influence the phenotype. A continuous distribution of measurements can be consistent with such contributions, but shape alone does not identify a particular gene count. Heritability is a population statistic under specified conditions, not the fraction of an individual trait caused by DNA and not a claim that environment is irrelevant. A single Punnett square may model one locus but cannot capture every influence on a complex trait.

Objective: Recognize epistasis and chromosome-linked inheritance.

Epistasis occurs when variation at one locus modifies or masks the phenotypic effect of another locus. A gene on a sex chromosome may show an inheritance pattern affected by chromosome complement, but sex determination and sex-chromosome systems vary among species and individuals; an X-linked recessive model is a specific simplified case rather than a universal rule. Genetic linkage concerns loci on the same chromosome, whereas epistasis concerns interaction of effects on a phenotype; the two ideas are distinct. Maternal inheritance through organelles can also depart from a simple autosomal model. Choose a model based on the organism and evidence.