Non-Mendelian Inheritance Patterns Study Pack
Kibin's free study pack on Non-Mendelian Inheritance Patterns includes a 7-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 27, 2026
Non-Mendelian Inheritance Patterns Study Guide
Unpack the genetics beyond Mendel's rules — from incomplete dominance and codominance to polygenic traits, pleiotropy, epistasis, and sex-linked inheritance — with the core mechanisms and classic examples you need for the AP Biology exam.
Key Takeaways
- •Incomplete dominance produces a blended intermediate phenotype in heterozygotes because neither allele fully suppresses the other's effect, as seen in red × white snapdragons yielding pink offspring.
- •Codominance differs from incomplete dominance in that both alleles are fully and simultaneously expressed in heterozygotes, producing a phenotype that displays both traits distinctly rather than blending them — the ABO blood type system is the classic example.
- •Multiple alleles expand a gene beyond the two-allele Mendelian model; a population can carry three or more alleles for a single locus, though any individual still carries only two.
- •Pleiotropy occurs when a single gene influences two or more seemingly unrelated phenotypic traits, meaning a single mutation can produce wide-ranging effects across different body systems.
- •Polygenic inheritance arises when multiple genes each contribute additively to a single trait, generating a continuous range of phenotypes in a population rather than discrete categories.
- •Epistasis occurs when alleles at one gene locus mask or modify the phenotypic expression of alleles at a different locus, distorting expected Mendelian ratios.
- •Sex-linked traits are encoded on sex chromosomes and therefore show different inheritance frequencies between males and females, with X-linked recessive conditions appearing more often in males due to hemizygosity.
Dominance Relationships Beyond Simple Dominance
Classical Mendelian genetics assumes one allele completely dominates another, but many genes follow different rules in which heterozygotes express a phenotype that is neither purely dominant nor purely recessive.
Incomplete Dominance
- •In incomplete dominance, neither allele completely masks the other, so heterozygotes show a phenotype intermediate between the two homozygous forms.
- •The snapdragon flower illustrates this: crossing a red-flowered plant (R¹R¹) with a white-flowered plant (R²R²) produces pink-flowered F₁ offspring (R¹R²) because each allele contributes a partial dose of pigment.
- •The 1:2:1 genotypic ratio in the F₂ generation corresponds to a 1 red : 2 pink : 1 white phenotypic ratio, a detectable departure from Mendel's 3:1 pattern.
Codominance
- •Codominance occurs when both alleles in a heterozygote are fully expressed at the same time, so the phenotype displays both traits simultaneously rather than blending them.
- •Human ABO blood typing demonstrates codominance: the Iᴬ allele directs production of A antigens and the Iᴮ allele directs production of B antigens; an IᴬIᴮ individual expresses both antigen types on red blood cells, yielding blood type AB.
- •Codominance and incomplete dominance differ in outcome: codominance shows both phenotypes distinctly present, while incomplete dominance produces a new, intermediate phenotype.
Multiple Alleles and the ABO Blood Group System
Many genes exist in more than two allelic forms within a population, expanding the range of possible genotypes and phenotypes beyond what Mendel described with his two-allele model.
Population-Level Allele Diversity
- •A multiple allele system means a single gene has three or more versions circulating in a population, even though each individual organism still carries exactly two alleles — one on each homologous chromosome.
- •The ABO blood group locus has three common alleles: Iᴬ, Iᴮ, and i, which together generate four possible blood types (A, B, AB, and O) depending on which pair of alleles an individual inherits.
Dominance Hierarchy Among Multiple Alleles
- •When more than two alleles exist, their dominance relationships form a hierarchy: Iᴬ and Iᴮ are each dominant over i (the recessive allele that encodes no surface antigen), but Iᴬ and Iᴮ are codominant with each other.
- •An individual with genotype Iᴬi has blood type A, one with Iᴮi has type B, one with ii has type O, and one with IᴬIᴮ has type AB — each outcome reflects a specific combination of dominance rules.
One Gene, Many Effects: Pleiotropy
Pleiotropy describes the phenomenon in which a single gene influences multiple, often unrelated, phenotypic characteristics, demonstrating that the relationship between genes and traits is rarely one-to-one.
Mechanism Behind Pleiotropic Effects
- •Pleiotropic effects arise because many gene products — enzymes, structural proteins, signaling molecules — participate in multiple developmental or physiological pathways simultaneously.
- •A mutation in a single pleiotropic gene can therefore disrupt several distinct biological processes at once, generating a syndrome with multiple symptoms from a single genetic change.
Clinical and Biological Examples
- •Sickle cell disease results from a single point mutation in the HBB gene encoding the beta-globin subunit of hemoglobin; the abnormal hemoglobin causes red blood cell sickling, which then produces wide-ranging effects including anemia, impaired circulation, organ damage, and increased susceptibility to certain infections.
- •Phenylketonuria (PKU) similarly stems from a single gene defect affecting the enzyme phenylalanine hydroxylase, leading to neurological damage, altered pigmentation, and other systemic effects.
Multiple Genes, One Trait: Polygenic Inheritance
Polygenic inheritance occurs when two or more genes each contribute incrementally to a single phenotype, producing a continuous spectrum of variation rather than the discrete categories Mendel observed in pea plants.
Additive Gene Action
- •In polygenic systems, each contributing gene locus typically has a dominant allele that adds a quantitative increment to the trait and a recessive allele that does not; the more contributing dominant alleles an individual carries across all relevant loci, the more extreme the phenotype.
- •Human skin pigmentation is controlled by several genes (including MC1R, SLC24A5, and TYR), each adding or subtracting melanin production; this additive system generates the continuous gradient of skin tones observed in human populations.
Continuous Phenotypic Distributions
- •Polygenic traits like height, weight, and skin color form a bell-shaped (normal) distribution in large populations because the probability of inheriting many contributing dominant alleles simultaneously is lower than inheriting an intermediate combination.
- •Because many environmental factors also interact with polygenic traits, the concept of multifactorial inheritance extends the model to include both genetic contributions and environmental influences acting together.
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Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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Question 1 of 25
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When a red-flowered snapdragon (R¹R¹) is crossed with a white-flowered snapdragon (R²R²), what phenotype do the F₁ offspring display?
Card 1 of 30
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Concept 1 of 5
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Incomplete Dominance vs. Codominance
Explain the difference between incomplete dominance and codominance in your own words. How do the phenotypes of heterozygotes differ between these two patterns, and what does each reveal about how alleles interact?
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