AP Biology: Non?Mendelian Inheritance – Incomplete Dominance, Codominance, Multiple Alleles, Polygenic

Non?Mendelian Inheritance – Incomplete Dominance, Codominance, Multiple Alleles, Polygenic

Concept Summary

• Incomplete Dominance: Heterozygous phenotype is a blend of the two homozygous phenotypes (e.g., pink flowers from red and white parents), showing that dominance is not always complete.
• Codominance: Heterozygous phenotype expresses both alleles fully and separately (e.g., AB blood type, roan cattle), demonstrating simultaneous allele expression.
• Multiple Alleles: A gene has more than two allelic forms (e.g., human ABO blood groups with I?, I?, i), expanding genetic diversity beyond simple dominance.
• Polygenic Inheritance: Multiple genes contribute to a single trait (e.g., skin color, height), producing continuous variation and a bell-curve distribution.
• Pleiotropy (Bonus): A single gene affects multiple traits (e.g., sickle-cell anemia), contrasting with polygenic inheritance where multiple genes affect one trait.

Core Questions

WHAT (definitional)

Q: What is incomplete dominance? A: A genetic pattern where the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes (e.g., red + white = pink). Trap/Clarification: Incomplete dominance is not blending inheritance—alleles remain distinct and segregate in gametes (Mendel’s laws still apply).

Q: What are multiple alleles? A: A gene with three or more allelic forms in a population (e.g., ABO blood types: I?, I?, i), though an individual can only inherit two. Trap/Clarification: Multiple alleles do not violate Mendel’s laws—each parent still passes only one allele per gene to offspring.

WHY (causal/explanatory)

Q: Why does codominance occur? A: Both alleles are fully expressed in heterozygotes because neither is recessive, often due to independent protein products (e.g., I? and I? produce different surface antigens). Trap/Clarification: Codominance is not the same as incomplete dominance—phenotypes are not blended but simultaneously visible (e.g., AB blood type, not a mix).

Q: Why is polygenic inheritance important? A: It explains continuous variation (e.g., height, skin color) and quantitative traits, which are influenced by environmental factors and multiple genes. Trap/Clarification: Polygenic traits do not follow discrete ratios (e.g., 3:1)—expect a bell curve of phenotypes.

HOW (process/application)

Q: How do you predict phenotypes for incomplete dominance? A: Use a Punnett square with uppercase/lowercase letters (e.g., R = red, r = white; Rr = pink) and no recessive masking. Trap/Clarification: Do not assume dominance—heterozygotes will always show a blended phenotype (e.g., Rr-red).

Q: How is blood type determined in the ABO system? A: Genotype-Phenotype: - I?I? or I?i = Type A - I?I? or I?i = Type B - I?I? = Type AB (codominant) - ii = Type O (recessive) Trap/Clarification: Type O is not "dominant"—it’s recessive to both I? and I?.

Q: How do you calculate the number of possible phenotypes in polygenic inheritance? A: For n genes with 2 alleles each, phenotypes = 2n + 1 (e.g., 3 genes-7 possible phenotypes). Trap/Clarification: This formula assumes additive effects—environmental factors can increase variability beyond the predicted range.

CAN (conditions/possibilities)

Q: Can a child have a blood type not present in either parent? A: Yes—if both parents are heterozygous (e.g., I?i × I?i), they can produce an ii (Type O) child. Trap/Clarification: Type O is not "hidden"—it’s a recessive phenotype that can appear if both parents carry the i allele.

Q: Can polygenic traits show discrete variation? A: No—polygenic traits always produce continuous variation (e.g., height, skin color) due to additive gene effects and environmental influence. Trap/Clarification: Discrete traits (e.g., flower color) are Mendelian, not polygenic.

Quick Facts & Traps

• Fact: Codominance vs. Incomplete Dominance: Codominance = both alleles visible (e.g., AB blood); incomplete = blended phenotype (e.g., pink flowers).
• Trap: "Type O is dominant"-Reality: Type O (ii) is recessive to both I? and I?.
• Fact: Polygenic traits often follow a normal distribution (bell curve) due to additive gene effects.
• Trap: "Polygenic = many phenotypes"-Reality: Polygenic traits have continuous variation, not discrete categories.
• Fact: Multiple alleles increase genetic diversity but do not change Mendel’s laws—each parent still passes one allele per gene.
• Trap: "Incomplete dominance breaks Mendel’s laws"-Reality: Alleles segregate normally—only the phenotype is blended.

Rapid-Fire True/False

• Statement: A roan cow (red and white hairs) is an example of incomplete dominance. Answer: FALSE Why the common mistake happens: Roan cattle show codominance (both alleles fully expressed), not a blend.

• Statement: A child with Type AB blood can have a parent with Type O blood. Answer: FALSE Why the common mistake happens: Type AB requires both I? and I? alleles, which cannot come from a Type O (ii) parent.

• Statement: Skin color is controlled by a single gene with multiple alleles. Answer: FALSE Why the common mistake happens: Skin color is polygenic (multiple genes), not a case of multiple alleles for one gene.