Non-Mendelian Inheritance
Beyond simple dominance.
Beyond simple dominance
Mendel got lucky — the pea traits he studied were unusually well-behaved. Most real inheritance is messier. Incomplete dominance gives blended heterozygotes (red × white snapdragons → pink). Codominance shows both alleles fully and separately (AB blood type expresses both A and B antigens; roan cattle show both red and white hairs).
Multiple alleles & ABO blood typing
A gene can have more than two alleles in a population (even though one individual still has only two). ABO blood type uses three alleles: Iᴬ and Iᴮ (codominant to each other), and i (recessive to both). Possible genotypes: IᴬIᴬ or Iᴬi (type A), IᴮIᴮ or Iᴮi (type B), IᴬIᴮ (type AB), ii (type O).
Sex-linked inheritance
Genes on the X chromosome (X-linked) follow special patterns because males have only one X. A recessive X-linked allele is always expressed in males (they have no second X to mask it) but only expressed in homozygous females. This is why color blindness and hemophilia appear far more often in men.
Mothers who are carriers (X^X^a) have a 50% chance of passing the trait to each son. Affected fathers pass the X-linked allele to every daughter (all become carriers) but to no sons.
Linked genes and recombination
Genes on the same chromosome tend to be inherited together (linked), which violates independent assortment. Crossing over can break linkage and produce recombinant gametes.
Recombination frequency = (recombinant offspring / total offspring) × 100%. The more often two linked genes are separated by crossing over, the farther apart they must be — this is the basis for genetic maps (1% recombination = 1 map unit, or 1 centimorgan).
Polygenic & multifactorial traits
Many traits — human height, skin color, eye color — are controlled by many genes at once (polygenic), producing continuous (bell-curve) variation rather than discrete categories. Environment (nutrition, sun exposure, etc.) layers on top of genetics to produce the final phenotype.
Epigenetics
DNA sequence isn't the only inheritable information. Chemical tags like DNA methylation (usually silences genes) and histone modification (acetylation loosens chromatin and increases transcription) can change gene expression and be passed to daughter cells — even to offspring in some cases. Diet, stress, and toxins can leave epigenetic marks.
Pedigrees
- Squares = males, circles = females, filled = affected, horizontal lines = mating, vertical = offspring.
- Autosomal recessive: trait can skip generations; unaffected parents can have affected children.
- Autosomal dominant: trait usually appears in every generation; at least one affected parent for each affected child.
- X-linked recessive: many more affected males; never father-to-son transmission.
- X-linked dominant: affected fathers pass to all daughters, no sons; affected mothers pass to 50% of all children.
Key terms
Quick definitions to lock in before the exam.